Yes, but using 20-50wt oil to maintain proper oil pressure, would indicate that clearances
were way off. If you don't need 20-50wt, because your clearances are good, and your oil
pressure is good, I would not use it. 20-50wt would also give poor protection at startup, because
it would flow more slowly. I've been using 0 -30wt synthetic all summer on my 496, and everything seems fine.
The only thing different I noticed was on real hot days, my oil pressure was only about 50psi, when otherwise
it would be 60 - 65.
There you go again!
You are trying to use your engine as an example without telling us how it was built, how hot your climate is, what oil pump is in the motor, or what rpm you turn it to. I recalled telling you that the light weight would be okay if your car maintained sufficient pressure at full throttle, maximum rpm, on a hot day.
For me, that would be a bit over 6000 rpm at 100+ degs. For you, it is probably something different.
If you look at the factory clearances and the factory suggested oil weights, you can pretty quickly determine why they spec'd the weight they did.
There are probably quite a few with these cars that have factory clearances because they have not rebuilt their engines or, if they did, the machine shop looked at the books and built them that way. If they put a hv pump on the engine, then they flow more oil which should produce a higher pressure with all factors being the same.
Engine building has all to do about it. If one has tight clearances, then with a given pump, one should have more pressure on a given oil. If one radiuses the oil passages, then flow increases and pressure drops IF the clearances are large and the pump does not have the volume capability to supply what the internal engine can move.
A 10-40 oil, on a cold start, should flow the same amount oil as a 10-30, but a bit less than a 0-30 unless the ambient temperature is below zero.
On an engine with strong valve springs, the pressure will try to force oil back out of a hydraulic lifter faster with a light weight oil than it will with a heavier oil because the heavier oil will resist moving out of the orifice in the lifter as compared to the lighter oil. In general, a light oil will drain down off the surfaces of the engine faster than a thicker oil. This should not be translated into thinking that I just said a thin oil protects less than a thicker oil. Good oils leave a film behind for months and months. Given that there is not a lot of load at start up unless one like to blip the throttle a lot, a few ticking lifters is not a sign of a problem if it last a few seconds and it may be that a thicker oil to some degree may actually reduce this because not as much got pushed out of the lifter in the given amount of time.
In the end, oil weight should be determined by the engine, ambient conditions and intended use.
No doubt in my mind that a lot of people run 20-50 because they think 100 psi of oil pressure is better than 60 psi on their 5000 rpm engine that has not seen more than 4000 rpm in three years.
