Synthetic to Conventional Oil

This post is to give me some insite on which oils can go back and forth with each other. For instance, in the winter its better to run a thinner oil and summer its better to run a thicker oil. I changed my car over to synthetic at 106k miles(i know not a good idea). It had burned oil before this and now it still burns oil, but I think I made the mistake of going with a fully synthetic instead I should have gone with a blend or a high mileage formula. Does anyone know if I can go from my synthetic to a high mileage oil and does high mileage oil contain a blend of synthetic and conventional oil? It is just confusing and I heard at one point in time that once you go synthetic you cant go back... is this true? I currently am running an amsoil 10w30 full synthetic oil that goes for a year service or 25k miles for new vehicles or for vehicles with 75k or higher 6months or 15k miles. Thank You anything will help.

This post is to give me some insite on which oils can go back and forth with each other. For instance, in the winter its better to run a thinner oil and summer its better to run a thicker oil. I changed my car over to synthetic at 106k miles(i know not a good idea). It had burned oil before this and now it still burns oil, but I think I made the mistake of going with a fully synthetic instead I should have gone with a blend or a high mileage formula. Does anyone know if I can go from my synthetic to a high mileage oil and does high mileage oil contain a blend of synthetic and conventional oil? It is just confusing and I heard at one point in time that once you go synthetic you cant go back... is this true? I currently am running an amsoil 10w30 full synthetic oil that goes for a year service or 25k miles for new vehicles or for vehicles with 75k or higher 6months or 15k miles. Thank You anything will help.


YEShttp://www.automotiveforums.com/vbulletin/images/smilies/grinyes.gif High milage oil can be synthetic, or blend, or conventional. You can switch to whatever from whatever. With the car burning oil, save your $$ and stay with conventional. I suggest you goto a 40 grade. Valvoline, Castrol, etc.

The thicker grade helps the older engine run better. I use 15w40(Delo 400) in my old '91 Ford and '74 Chevy. I live in California and the weather is mild.A Wisconsin man told me most people use 10w30 in that part of the country.


There is a ton of information, testimonials, specs, out there. Very confusing unless you are a lubricating engineer. Use a good filter NAPA gold, K&N. Synthetic will still collect carbon deposits and metal bits over time so to get your $$ worth go at least 7500 to 8k between changes. 25k is extreme I wouldn't do it.

Use the proper weight for your engine's age, bearing tolerances, and driving, but you can switch back and forth from any oil at any age. The synthetic/conventional myths are just that... myths.

I personally use 10w30 in nearly everything I drive. The best indicator of proper oil weight is oil pressure. An engine that required 10w30 new might be helped along in its autumn years by using 15w40 if the bearing tolerances have worn out allowing pressure to drop.

An oil that is too thin will let too much oil by the bearings allowing pressure to drop and not be there to protect the bearings. Oil thats too thick will not only be too high pressure, it won't be able to flow into the bearings fast enough and have a similar effect.

If you don't have one already, install an actual pressure gauge. 10 psi per 1000 rpms is the minimum. 80 psi is where oil filters can blow, or at least blow the O-ring off the filter flange. Its also where the bypass valve opens on most engines, so if you're constantly operating at or near the bypass pressure, the valve is bypassing the filter which isn't good.

Most of my engines never see more than 6000 rpms. They all operate at about 20 psi at idle, 30-40 psi on the highway, and never more than about 70. (those are hot pressures, by the way... cold will pretty much always peg it at bypass pressure)

Also, don't assume that because your engine has higher mileage that it NEEDS high mileage oil. Driving style and maintenance make engine life very variable. You might be worn out at 100k or just fine at 300k. Most oils are gimmicks. High mileage oils typically just have a bit more calcium compounds to neutralize blowby gasses. High HP oils and high TQ oils are usually just on the thin side of their advertised weight to coax an extra .05 hp out of the engine. If you notice, the ads for the high hp oil don't say they MAKE any hp, they say, "make sure you're getting all the horses you paid for," and "no other leading oil makes more hp." They never say that their oil improves anything.

I suggest you just run the proper weight, regardless of synthetic or regular and make sure to change it regularly.

I switched over to Amsoil fully synthetic on the Astro 4.3L at 75,000 miles. I now have 150,000 on the vehicle. I change oil and filter once a year. Last year the oil had 26,000 miles on it. I use 0W30. Every six months I send in a sample to Oil Analyzers to measure oil contamination, TBN (total base number - ability to neutralize acid), viscosity, and additives. I have done this consistently. Oil Analyzers graphs your samples so you can see trends in engine wear. It is very cool. My engine used approx. 4 oz. of oil every 1,000 miles at 75,000, and now, at 150,000 still uses about the same. Since switching over to synthetic I have doubled the miles on the engine, but have had very, very little engine wear.

It took years for me to be sold on the concept of extended drain interval. It is a new way of thinking about your oil. But with oil analysis, I am very comfortable leaving my oil in for much longer than 3,000 miles.

V6

2002 Chevrolet Astro
2006 Buick LaCrosse

I think curtis said it best.

I would like to add that Mobil 1 is no longer a full synthetic. It is now a group III oil much like Castrol Syntec. If you want a fully synthetic then I'd suggest Royal Purple or Redline.

As for the conventional/synthetic myth. It is just that, a myth. You can switch over at any time. Some people swear is has caused their older cars to leak. I don't buy that crap and if that were true then the problem didn't lie with the oil, rather it lies with the seals. They would have to be bad regardless.

Also, the only time I would not run a synthetic oil is during engine break in. The synthetics don't allow the rings to seat properly during the initial break in. I would run it for about 20 miles to clean any excess crap out from machining, change the oil with conventional and run for 3,000 miles, and then I would change it again with synthetic.

Just an FYI.

Check this out very good and long. http://www.vv.corvair.org/pipermail/virtualvairs/2005-July/028037.html

Sometimes switching to synthetic will cause leaks, but that is typical of older american cars that use cork and paper gaskets. The old gaskets crack and seep. Most of the cracks get filled with gunk, then running synthetic cleans out the gunk causing leaks.

I switched to synthetic on an 87 Olds with 60k and it leaked horribly, but I switched to synthetic on a BMW with over 200k and never leaked a drop.

I think curtis said it best.

I would like to add that Mobil 1 is no longer a full synthetic. It is now a group III oil much like Castrol Syntec. If you want a fully synthetic then I'd suggest Royal Purple or Redline.

As for the conventional/synthetic myth. It is just that, a myth. You can switch over at any time. Some people swear is has caused their older cars to leak. I don't buy that crap and if that were true then the problem didn't lie with the oil, rather it lies with the seals. They would have to be bad regardless.

Also, the only time I would not run a synthetic oil is during engine break in. The synthetics don't allow the rings to seat properly during the initial break in. I would run it for about 20 miles to clean any excess crap out from machining, change the oil with conventional and run for 3,000 miles, and then I would change it again with synthetic.

Just an FYI.
Thanks for putting one myth to rest, however you are perpetuating another! If engine break-in was a problem with synthetic oil, high line vehicle manufacturers would not use a factory fill of synthetic engine oil. Personally I have always used synthetic on new engines and rings always broke in just fine.

Thanks for putting one myth to rest, however you are perpetuating another! If engine break-in was a problem with synthetic oil, high line vehicle manufacturers would not use a factory fill of synthetic engine oil. Personally I have always used synthetic on new engines and rings always broke in just fine.

Hmm, I've gathered that from people that rebuild engines. I've also been told that manufactures pre-break in engines and they don't use synthetic.

That's just what I've been told though.

Also, just to reiterate. I've switched two cars with over 100,000 miles to synthetic without problem. I forgot to put that in.

With regard to new engines my understanding is it depends on the engine, the tolerance levels during manufacture and the materials used.

Certainly for older, iron block engines which might get a hand hone'd rebore and new rings then using synthetic oil is not a good idea.
These engines literally need to be worn in so everything seals and find it's place.

I imagine a brand new car, made with different materials and much tighter machining tolerances doesn't need to wear as much, if at all.

Ah, very good points. That makes a lot of sense.

I don't think you can generalize that switching from conventional to synthetic is a myth or is an old american car killer. I had a 95 eagle vision with 485000 Km on the original engine with original gaskets. To test the myth, I switched to synthetic which busted the valve cover gaskets and they started leaking oil. So I swapped the gaskets and everything was fine. So it could be damaging to switch to synthetic but it's such an easy fix that i don't think it matters all that much anyways. Going back to conventional oil theoretically shouldn't do any damage what so ever. If anything it would reseal any gasket cracks that were cleaned out by the synthetic oil. Some people that drive taxis around here 'clean' their engines by running synthetic every 3 or 4 oil changes. They change oil ever 2 weeks, so they can't afford to keep synthetic all the time.

I don't think you can generalize that switching from conventional to synthetic is a myth or is an old american car killer. I had a 95 eagle vision with 485000 Km on the original engine with original gaskets. To test the myth, I switched to synthetic which busted the valve cover gaskets and they started leaking oil. So I swapped the gaskets and everything was fine. So it could be damaging to switch to synthetic but it's such an easy fix that i don't think it matters all that much anyways. Going back to conventional oil theoretically shouldn't do any damage what so ever. If anything it would reseal any gasket cracks that were cleaned out by the synthetic oil. Some people that drive taxis around here 'clean' their engines by running synthetic every 3 or 4 oil changes. They change oil ever 2 weeks, so they can't afford to keep synthetic all the time.

So, you're saying that your gaskets with 485,000Km busted and you're surprised? I'm surprised it lasted that long. Like I said, the oil didn't cause the leak, and ass old gasket did. Also, can you prove that the oil caused the leak? Nope, I'll bet you can't. Also, for your reference her are the cars I've switched to synthetic all with no leaks before and after:

1989 Chrysler LeBaron GTC TII: 142,000mi
1989 Chrysler LeBaron GTC TII: 88,000mi
1994 Eagle Vision TSi: 112,000mi
1998 Chrysler Sebring LXi: 74,000mi
1991 Dodge Stealth R/T TT: 86,000mi

Hmm, I've gathered that from people that rebuild engines. I've also been told that manufactures pre-break in engines and they don't use synthetic.

That's just what I've been told though.

Also, just to reiterate. I've switched two cars with over 100,000 miles to synthetic without problem. I forgot to put that in.
Todays engines require very little break-in due to the precise control of tolerances and machining processes. Engines are not pre broken-in by the manufacturer. Most of your high line cars come from the assembly plant with synthetic lube, cars such as the Corvette, many Cadillacs, Dodge Viper, Porsche and many others. A properly re-built engine, be it old tech or hi tech, will break-in just fine on synthetic oil. The problem is that there are too many people that keep passing on bad information about many automotive subjects that no one knows what to believe. I have been involved in many hundreds of engines run on dynometer durability cycles and many fleets of vehicles that ran synthetic engine oil and not one of them failed to break in properly and quickly. The lack of excessive wear to internal engine parts was also amazing.

When I opened up a 1972 Cadillac 472 with 98,000 miles, the cylinders still had cross hatch pattern on them. I don't think breaking it in with synthetics would have made it wear less, or not break in properly!

So, you're saying that your gaskets with 485,000Km busted and you're surprised? I'm surprised it lasted that long. Like I said, the oil didn't cause the leak, and ass old gasket did. Also, can you prove that the oil caused the leak? Nope, I'll bet you can't. Also, for your reference her are the cars I've switched to synthetic all with no leaks before and after:

1989 Chrysler LeBaron GTC TII: 142,000mi
1989 Chrysler LeBaron GTC TII: 88,000mi
1994 Eagle Vision TSi: 112,000mi
1998 Chrysler Sebring LXi: 74,000mi
1991 Dodge Stealth R/T TT: 86,000mi

You're right. I can't prove that the oil busted the gasket because the gasket can bust at any given time. But when a gasket busts after 200 Km of changing to synthetic, I would assume that it was the oil conversion. Besides, most people don't replace a gasket before it starts leaking because they're not enthusiasts like you. LIKE I SAID, it's not going to blow your engine, but it will cause a leak if the gaskets are weak. It may not happen on all cars, but it does happen on some. BTW, all the cars you're talking about, with the exception of the first one, don't have high mileage. My eagle vision ran without any need for gasket changes until 450000 Km (not even the head gasket). My impala's engine died at 350000 Km and the gaskets are still fine. My buick is at 200000 and the gaskets are in good condition. So the oldest car you've ever done a conversion on has mileage of 230000 Km. The gaskets in that should be OK. That's most likely why they didn't fail from the conversion. Don't talk about your personal experience and call it fact, just like I'm NOT saying that changing to synthetic will blow your gaskets.

When I opened up a 1972 Cadillac 472 with 98,000 miles, the cylinders still had cross hatch pattern on them. I don't think breaking it in with synthetics would have made it wear less, or not break in properly!

I cracked open a 75 Caddy 500 with 223,000 on it and there were still crosshatches at the bottoms of the bore, and the ridge at the top wouldn't catch a fingernail. The caddy 425/472/500 family used super high-nickel blocks. Great for long term reliability, terrible for revs.

In all actuality, synthetic oil isn't really any "slipperier" than conventional. People ascribe qualities to it that don't really exist because its supposed to be superior. Oil is slippery, so they assume the expensive synthetic oil is slipperier. Its just not the case. GM shipped several hundred thousand corvettes (with and without roller cams) with Mobil1 in the crankcase and no break-in other than driving it off the assembly line to the shipping lot. I've toured the Bowling Green factory, and there isn't a break-in section. Many of the vettes we got at the dealer had 3 miles on them, which is about the distance from the factory to the old lot.

Oil is one of the most misunderstood parts of an automotive engine. I have to say that I've studied it for years and I still don't know as much as I'd like to... and half the stuff I do know for a fact there are a thousand old-school gearheads that scream at me for being a liar.

So... I'll leave the research up to the individual. :)

You're right. I can't prove that the oil busted the gasket because the gasket can bust at any given time. But when a gasket busts after 200 Km of changing to synthetic, I would assume that it was the oil conversion. Besides, most people don't replace a gasket before it starts leaking because they're not enthusiasts like you. LIKE I SAID, it's not going to blow your engine, but it will cause a leak if the gaskets are weak. It may not happen on all cars, but it does happen on some. BTW, all the cars you're talking about, with the exception of the first one, don't have high mileage. My eagle vision ran without any need for gasket changes until 450000 Km (not even the head gasket). My impala's engine died at 350000 Km and the gaskets are still fine. My buick is at 200000 and the gaskets are in good condition. So the oldest car you've ever done a conversion on has mileage of 230000 Km. The gaskets in that should be OK. That's most likely why they didn't fail from the conversion. Don't talk about your personal experience and call it fact, just like I'm NOT saying that changing to synthetic will blow your gaskets.

What I am trying to say is that the oil did not cause your gasket to burst. It was on it's way out. Like you said they can go at any time but they should be replaced after a certain mileage just like things like timing belts. I wouldn't go around just waiting for stuff to break before replacing them since they can cause other problems, more expensive ones and leaky valve cover gaskets can cause fires.

I'm not trying to pass my opinion off as fact. What I am trying to pass off as fact is that synthetic oil will NOT blow a seal or gasket. Perhaps it was coincidence. Perhaps the PCV wasn't working right. Maybe it was because your VC rubber seal was so damn old it was going to burst regardless. Chalk it up to coincidence. My current LeBaron's VC gasket started leaking shortly BEFORE changing to synthetic and it only had 87,000 miles on it.

So, take that for what it's worth.