You should run the thinnest oil that gives you sufficient oil pressure for the RPM you intend to run. That depends on your engine. What others run is really irrelevant to your engine.
The reason I ask is because the machine shop said 20-50. Told him I had 75 psi on break in with Brad Penn break in oil. I carry 35 with my current oil (hot) and around 80 at 5800
I didn't want or think I needed 20-50. The machine shop was telling me I needed it. I went to another shop and bought my oil. I've done a lot of reading on this site and knew what to use. I just wanted to see what others ran.
Your bearing clearances can dictate what oil will give you decent pressure. I focus more on oil volume around the big mains. When my first stroker was built in 1998,it was setup with additional clearance,and even before that engine was assembled,I knew I would have to run 20/50 to have any pressure. It would be about 60psi at cold startup. Once hot,it would be 15psi at hot idle. Many years back,I did try 15/50,and I had about 5psi at hot idle. That engine stayed in the car for 19 years. It was just removed this last spring,to go into another car. I did tear it down to inspect everything,and all my bearings and journals were mint. No bad surprises. My new engine has different clearances,per the builder,and this one runs 0-W20. Completely different animal here.
It isn’t a one-hit wonder. There are a bunch of similar builds to mine that are running this same oil. These are engines that are make 750-850 HP,and getting a lot of street and track time. A few different brands of billet cranks,but all have GRP Pro-material rods,with 2.00” rod journals,and a bunch of stroke.
I think the lesson here is about offering a suggestion on a builder's specific strategy, and that the entire question can't be as simple as "what oil?" without a very detailed build plan. Just as silly as a machine shop suggesting 20/50 without a specific explanation and a safe way to justify it's use is the blind assumption that there's only one way to build a Buick and a narrow margin of oils that would keep it alive. Use the oil type that follows your build paradigm. As the componentry changes, the clearances change depending on how much flex it might have to deal with. Generally speaking, stock GM rods need more flex room than 4340 and other lighter weight rods. Aluminum grows, everything changes as your build does.
I was using VR1 but I lost a cam lobe after 4,000 miles. Not saying it was the oil’s fault but I’m not taking chances. I switched to driven, I was thinking about going back to penn grade. I had really good luck with it.
I would use a 10W-40, like HR-5, https://drivenracingoil.com/i-30497806-hr5-10w-40-conventional-hot-rod-oil.html GP-1 looks good. 10W-30 might be too thin. You can try it and see how your hot oil pressure is. https://drivenracingoil.com/c-1389373-shop-by-product-gp-1-engine-oils.html https://drivenracingoil.com/i-30497795-gp-1-10w-30-synthetic-blend-high-performance-oil.html
I ran Brad Penn 10w40 (Penn Grade 1) for many years with no issues...that I know of. Last year I used Amsoil Z-Rod 10w40, again worked fine. This year I am using some Royal Purple HPS 10w40, hopefully the engine does not explode.
