5w30
I have Mobil 1 in the 292 right now.
I would love to use Rotella or Delo because they have (or did have) more zinc, but only if they had 5w30 or even 10w30.
20w50 is good for race engines that are built a little loose and torn down after a race.
Most engine wear occurs during engine warm up. 20w50 and STP would starve an engine for oil during warm-up and really srain the distributor gears. Modern engines have fast warm-up and run 5w20 and even 0w20. Fast warm-up helps with emissions as well.
I can't imagine how hard 20w50 and STP would be on an engine.
2009-1978=31 years of that combo. How long have you been driving yours? They haven't even made Mobil 1 that long. The whole point of zinc is that it adheres to the metal and helps on cold starts. Brand new GMCs used 50w. The guy asked what I used. I told him. This truck has more real hard miles on it than any other vehicle I have ever owned and probably any you have either. It's a daily driver and sleeps outside all year. It fires up at -15 and hauls hay and fire wood up a bad steep road when it's 110. It did a 4,000 + road trip to Lone Star Roundup In Austin. Most of it 75-80 mph. By the way the zinc in your diesel oils is gone too.
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