synthetic oil

[top3mcm]

Someone told me that once you were using synthetic oil that you shouldn't put regular oil back into the vehicle, does anyone know if this is true or not?

[ivymike1031]

sounds like a fish tale to me...

[BeEfCaKe]

I think synthetic oil contains mostly natural oil still..

[911GT2]

Other way around I've heard.

Synthetic doesn't soak into the gaskets and expand nearly as much as conventional so putting syn into an engine which has had conventional for a long time may suffer from gasket leaks.

[454Casull]

You can mix and match synthetic and petroleum lubricants in any ratio. However, you probably don't want to add synthetic to an old engine, as the aforementioned leaks may happen.

[fatbot225]

Awhile back I decided to use Synthetic oil in my sister's '97 Cavalier. She was about 200 miles overdue her regular 3k oil change and i thought synthetic would help clean out the engine a little but after awhile it started leaking from the valve cover gasket.

I figure synthetic isn't worth the extra money. I just switched back to regular oil and change it every 2,000-3,000 miles depending on what brand oil is on sale.

[SaabJohan]

Today many cars have oilchange intervals from 10000 miles and higher. To achieve that a synthetic oil is recommended.

Synthetic oils are better in almost everything; the only downside is the price.

Synthetic oils are chemically engineerd, but the raw product can be crude oil just like for regular oil.

If you buy a syntetic oil with a mineral baseoil then you have been fooled. However, 100% of the oil is not necessary synthetic.

Regular and synthetic oils have the same cleaning addetives so synthetic oils do not clean any better than a modern regular oil. They are also not responsible for oil leaks. If your engine starts leaking after an oilchange it would also have done so with regular oil.

[Dillweed]

You usually have up to 60,000 miles to switch from regular oil to synthetic.
However, you can switch from synthetic to regular oil whenever.

Here's what happens with the leaks. After around 60,000 miles your seals and gaskets start showing minor signs of wear, due to the wear of the engine. Regular oil is thicker and less cleansing let's say so it doesn't leak 'cause it creates deposits.

Now...
If you switch from regular to synthetic oil AFTER SIGNIFICANT MILEAGE, the synthetic cleans away the deposits and reveals the imperfections and wear of the seals and gaskets. And thus being thinner oil and more cleansing it leaks through.

BUT...
If you start early with synthetic treatment, your engine, seals and gaskets wear way less fast and in turn you can run synthetic 'till kingdom come.

Hope I helped clearify the subject a little.

[ivymike1031]

I have several complaints with the old "synthetic causes leaks" tale:

1) If you've ever tried to stop a damaged seal from leaking by applying a sealant, you probably realize that it's next to impossible. I have trouble imagining that the tiny bit of scum that old motor oil leaves behind is some sort of miracle sealant.

2) I know many engine builders who have tested the "synthetic causes leaks" theory, and none have ever been able to demonstrate that it works in practice.

On a separate subject, synthetic oil is not necessarily thinner than conventional oil. If the viscosity grades written on the bottles were truly indicative of the viscosity of the oil in the bottles (which they aren't), then a 5W-30 synthetic would be exactly as "thick" (viscous) as a 5W-30 conventional oil. In reality, a good synthetic oil will have a higher viscosity at high temps (when it counts) than a similarly rated conventional oil, while offering a lower viscosity than the conv. oil at cooler temps (good for fuel economy).

According to a gentleman at Mobil 1, their 0W-20 synthetic has the same viscosity as a conventional 5W-30 oil has at 150degC (about 2.7 CP). The 0W-20 oil offers substantially improved behavior at cooler temps, as the viscosity doesn't increase by nearly as much (more power output, better fuel economy).

I've heard that there are -5W20 and -15W20 synthetic oils coming soon, which will offer a viscosity that hardly changes with temperature. I'll be interested to see how that works out. You'd be looking at about 3% fuel economy improvement with one of those oils (vs 10W40), by my own estimate.

[FYRHWK1]

well, using synthetic on high mileage engines with any normal amount of ring wear will get you oil burning issues since the molecules of synthetic are smaller and more uniform. Synthetic MAY help reduce these if used from the start so it's possible that switching to synthetic after running organic will cause oil consumption, but it'll happen to a car that ran synthetic from day 1 when it starts to get wear on its rings/valveguides/etc, that point jjutst may be further odwn the line since synth protects better.

[ivymike1031]

The oil film thickness that passes by a piston ring is WAY thicker than the size of an oil molecule (by a factor of 20-50), so I don't see how the size of the molecules will make a heck of a lot of difference. I'm not sure what the uniformity of the size of the molecules has to do with this either. Care to straighten me out on this?

[FYRHWK1]

the film is much thicker, but the smaller and more uniform the molecules the more will slip by there, also the same with the valveguides, I suppose the difference really isn't large being there'll be missed molecules regardless, but the larger they are the more likely they'll be caught, especially with something like a valveguide where gravity is drawing the oil down, if there's any wear the smaller particles will flow right through it. Uniformity, i guess it really doesn't matter.