On oil, viscosity, and burned exhaust valves

[MrPink]

In another thread, Woodie and I were disagreeing on oil for our beloved trashy cars. Since I did not want to take the original thread off topic, I have started this one. Feel free to jump in! I am not going to quote, I will paraphrase, so don't jump on me for unintended nuance error.

On 5W-30 vs 10W-30 oil:

Woodie says that we should be running the 5W stuff in our cars, because everyone knows that thinner oil is better for power, economy and other things. I don't agree with this. More importantly, he says that Suzuki decided that 5W-30 oil is better than the 10W-30 oil because of this burned exhaust valve phenomenon, something I have never experienced, never heard of before (not saying it isn't so, simply saying that I have not heard of it) and cannot find much more than anecdotal evidence for.

Here's my confusion:

As I understand multigrade oil chemistry, basically what one does is start with the thin base oil at whatever the W viscosity is, and add various amounts of magic fru-fru (otherwise known as polymers) to make the oil behave as if it was a thicker oil at operating temperature.

The wider the multigrade spacing, the more magic fru-fru there is in the oil to make it do its trick. However, fru-fru isn't nearly as good at lubrication as base stock is, and it breaks down fast. A 5W-30 oil pours like 5W at 0-degrees (because it is 5W oil) and behaves like 30-weight at 100 degrees because of the fru-fru. However, all things being equal, the 5W-30 will break down faster than the 10W-30, because again, fru-fru isn't particularly lubricious or long-lived, and there's more of it in 5W-30 than there is in 10W-30.

At operating temperature, the two oils should be the same, since that's the whole point of this game. Why Suzuki would say that 5W-30 is preferred is a little confusing to me, since unless you live in Fargo, you probably don't need a 5W oil to start the car in the morning. Plus, the fact that an oil change interval must necessarily be shorter with a 5W-30 vs a 10W-30 oil because the 5W can't live as long also seems counter-intuitive to me. The last sticking point is that there are 10W oils with lower pour-points than some 5W oils, which absolutely means that your car will turn over easier at the North Pole with one of them in the crankcase than the 5W oil.

Huh.

That last thing I would like to know more about is this burned valve phenomenon and how it relates to thicker oil. I never had a problem with my old 3-beastie, and I don't seem to be having the issue with my current four. I don't understand how thicker oil can make this difference, unless the oil pump is so pathetic that it can't pump thicker oil, and people have thick oil and redline their engines when it's 0-degrees out and they start the car.

Enlighten me, please, and add your twenty-five cents (hey, inflation!) worth.

-Pink

[Woodie83]

The thing that Suzuki decided was partially at fault for the universal burned exhaust valves in G10s was the second number. They originally said 10W-40 was okay, the "fix" was to drop that and recommend only 5W-30 or 10W-30 with 5W-30 preferred. After the original cars started burning valves en mass, they looked into it and found that the thick oil wasn't draining back out of the lifters, causing the lifters to overpump and hold the valves open. There are many things which conspire to burn valves, the main one being late ignition timing which is a maladjustment put there solely to reduce NOx emissions.

You're right that wider oil viscosity spreads in dino oil are achieved chemically and reduce oil longevity, but I never use anything but synthetic so I don't worry about that. Plus the recommended oil change interval is 7500 miles, if you're nervous, reduce that to 5000 and drive away happily, that's still fairly long.

[older'n'dirt]

Comments for your thread:

It is my opinion that there are two parameters for selection of engine oil. The first being that any multi-grade, whether "natural" or synthetic is no good. There is nothing wrong with running straight grade oil in your engine. The second is that there is only one kind of oil to use and that is oil refined from Pennsylvania crude.

[doug7x]

Now, I do understand the "not draining fast enough" burned valve theory, & although this makes sense, it is NOT the reason to use the lightest possible viscosity oil. Fact is, 99% of engine wear occurs during cold starts (by "cold", I mean the engine not at operating temp, not just the outside air temp.) The thinner the lube (lower viscosity), the sooner it will be pumped to where it needs to go, thus lubricating all of our moving parts sooner. With the synthetic blends available today, the film strength is far superior to the stuff we used years ago. ( I used to SWEAR by Quaker State 20w40 deluxe...I could easily get 120-150,000 miles on my old small block Chevys...Now I'm using Valvoline 5w30 dura-blend, & putting twice the miles on an engine turning 2 or 3 times as many RPM on the highway. I'd be willing to bet that if I had been able to use this stuff back in the 70's, those old SBC's would run over 1/2 million miles) Hope this helps

[MrPink]

Hmmm... One could also make the case that the thinner the oil, the faster it drains to the bottom of the crankcase, leaving metal parts bare at start-up.

Of course, adding MPZ solves this problem.

-Pink

[doug7x]

Well, Mr Pink, the only way I can think of to try change your view would be to conduct a little experiment. Take 2 identical metal parts that you have laying around. (could be anything...carbon steel would illustrate this the best) Maybe 2 old piston rings. Dip one of them in 40w (or whatever you think would last longer), dip the other in 5w30 synthetic (pure or a blend...doesn't matter) Hang both rings side by side on nails in a humid place (your garage or basement), & give this a few weeks...the first ring to show rust will confirm or disprove your theory. I'm NOT sure which one will win this, but I'd bet it'll be pretty close.
Even if the "runs off faster" theory is correct, I still want lube ASAP when I'm starting cold. Come to think of it, why do YOU think our engines are lasting longer & longer, if not for the advances in lubrication. I mean, babbit bearings & iron cranks, steel rings in iron bores have not changed much, but I remember when it was common practice (and necessary) to do a complete ring & valve job at 50, 000 miles. Also, think about these new hybrids thst start & stop the engines constantly...the Prius (at least the 1st couple yrs) requires ZERO w30......( NOTE....conducting this experiment COULD require the drinking of MANY cold 12oz bottles, what with watching for rust & all...I think I'm going to do it also LOL)