My question to you would be, and you being the oil expert, you should have a good guess”
I like the fact that people here hold my knowledge and opinions about oil/lubricants in such high esteem, but compared to some of the folks at BITOG, I’m a lowly noob.
Still, I’ll try to address the questions you’ve asked with what I know.
“At what point does oil become too thick or too thin to adequately protect? I know in part it relies on the clearances in the engine, but some size engines just simply shouldn't use above or below a certain weight oil due to hydrodynamic loads.”
You are on the right track. Truth is, most automotive engines are OK with an oil in the mid 20 weight to mid 40 weight range in most conditions. Any differences in wear will probably not be noticeable … maybe not even with careful UOA monitoring.
“I mean, can you conceivably cause excess bearing wear simply by using too thin an oil, or is that pretty much impossible?”
Nope, that’s always been a concern … and I think I’ve seen some Chevy V8s using mass-market 5W-30 and even 10W-30 which had thinned and showed high lead levels. However, you are not likely to get into trouble using the factory recommended weights unless you push the interval too far. Current mineral 5/10W-30 oils seems to thin out … even the better ones like Pennzoil and Chevron. Castrol is/was worse than those two and Royal Purple
synthetic 10W-30 will thin down to a 20 weight in 1,500 miles of typical street use.
“Is it also possible to cause wear by using too thick an oil and placing greater strain on the oil pump and bearings ...”
Again, we at BITOG (especially
Terry Dyson) think we’ve seen examples of this. However, you’d have to be talking about using 15W-40 in a new/tight motor … or 20W-50 … or even a straight 40 or 50 weight.
But even then, some manufacturers like BMW recommend
very thick oils (10W-60
) for some of their higher-performance motors.
So, to answer your question, it all depends.
