I have been using Brad Penn, now Driven GP1 20w50, I also use 15w50 VR1 racing oil and Brad Penn 15w40 diesel oil that I put in my truck. Now que all the moans about thick oil in a regular car engine. All my cars have stiffer valve springs, .470- .557 lifts. The mellowest one can and does get stuck in traffic idle time is high, it also has overdrive which it lumbers along approximately 2k at 60-65. I don't do many short trips, store, bank, auto parts is the only close things to my house. I do not use 10w30 in anything, I consider that winter oil. 10w40 is becoming a obsolete oil, still a 10weight oil, but protects warm like a 40. New cars are built different, they don't contaminate oil as bad and they know if you up the weight, so I follow recommended on them. Sure I may be wasting some HP on turning the oil pump with thicker oil. I also don't put engines together with high volume pumps unless rpm is going over 6k and full grooved main bearings. All my junk is measured torqued and dropped in used stuff, no machine work (except Barracuda), they all hold 25+psi at heat soaked idle and 50+ at speed. I'll call that good. I say put whatever oil you feel good with as long as it has high zinc from manufacturer, with good detergent package for our stinky engines. Adding zinc to modern oil is not a great idea as mentioned above the detergents negate the zinc like it is a contaminate. Let's face it your driving old technology you need old technology oil. I guess I would just not recommend 10w30 or less in our clearanced out worn/leaky engine with a carburetor. Anything above that is fine I think.