So the people that know what oil is “best” (and best being a broad term) are of course the people that specialise in the engine. RPi, John Eals and V8 developments to the best of my knowledge all recommend a 20/50 with a high zinc additive (ZDDP). Then you turn to the forums / clubs, again 20/50 with a high zinc addative is the common answer, typically Valvoline VR1 20/50. Some run 10/40 and realistically that’s not wrong for the engine if you read the workshop manuals, although it’s a little thin in some peoples eye. Coming back to the term “best” this is a very subjective term. What is best for one person may be different to what is best for someone else, climates come into play along with the build specification of the engine and its use. Although I say climates come into play I have always countered that argument by saying although the climate might be cold the engines running temp is still very much the same when at working temp.
I often get asked what oil would I use if Valvoline VR1 was not available, often because it is out of stock with many suppliers at the same time and sometimes becuase of price!
From what I have read (or at least can find on the internet) VR1 has between 1300 and 1400 PPM ZDDP. Westway oils do a 20w/50 with a high ZDDP engine oil. Again from what I can find on the internet this has a ZDDP level of 1500 PPM. So in comparison its looking good. For that reason I have now switched to the Westway oil and have just done an oil change in the twin turbo P38 using it. Is it any good you ask? It is far to early to tell, realisticly I will never know, the odometer is reading 164,000 miles (January 2024), the engine has done around 32,000 miles since rebuild (July 2018), 9,000 of those with the turbos fitted (June 2021). If the cam wears out next week then for sure the oil will be a big question mark however if the camshaft lobes or crank bearings etc wear out in 70,000 miles time can I realisticly say that would not have happened if I was still using VR1. Nope.
Unless you measure things for wear and then run the engine for 100k miles and check for wear again and then do the same test with a different oil however starting from the same point 100k miles ago in terms of wear on the engine and then compare there is no real answer to whats the best oil. It really just comes down to what the majority run and have good results with.
Heres a good one for you, I know of many engines (around 50 if not mopre) that were built to run on dyno test cells 24/7 to age exhaust components for testing. These engines ran on 10/40 with a zinc additive. They cover drive cycles with the equivalent load and rpm of towing a trailer between 50 and 80 ish MPH (from memory) and the engines still look spotless inside after 1500 to 2000 hours. 2000 hours at 55 MPH is 110,000 miles!!! The oil is changed frequently and the engine is at operating temp for most of its life. The honestly run for 23 hours a day and are only stopped to carry out checks and oil top ups.
So , my take from all of this is and it always has been the same. Frequency of oil change is far more important for cleanliness of the engine than the oil your actually using. The grade of the oil 10/40 or 20/50 is not realistically going to make a huge difference as long as the engine is in good condition and not being hammered or raced. The zinc additive is a good thing as that is going to help prevent wear. For sure if the engine is worn then you will very likely get a reduciton in noise etc by switching between 10/40 or 20/50 as you are increasing oil flow but decreasing potential pressure with a thinner oil and vice versa.
Im not sure if this ultimately answers the “What oil?” question or just muddies the water but hopefully it is of some help to anyone reading this.