I know that this subject has been beaten to death, but...

What is the best dino oil to use in our 4cyl OHC engines? I'm maintaining two b23's and a couple b230fs. The '85 and '86 have their crankcases filled with Castrol GTX 20w50 at the moment. The '83 has Chevron Delo 10w30 in it, and the '84 is doing time on Pennzoil 10w40. I'm uncertain about what weight I should be using. IIRC, Volvo recommends 10w30 for these cars. However, I live in the SF bay area, where the ambient temp rarely ever drops below 50 degress. I am self-employed and spend several hours on the freeway/highway everyday. I drove almost 65,000 miles last year alone. For these reasons, I've opted to switch to 20-50 on the cars that I drive the most, the '85 and '86 wagons. I figure that they don't need the cold weather protection that the lower weight would provide, and I think that the heavier oil provides better protection at the other end of the spectrum given my driving habits (high sustained rpm's, more heat generated by the engine, etc.). Is this generally a bad idea? My only concern is that I might be gumming something up with the heavier oil. Will it put more stress on the pump/sump? Saw something in the archives about only using 20w50 if the engine had an oil consumption problem (not the case). If it's really too thick, what is this going to do to the engine?

The best dino oil I've ever used is Castrol GP50. I used it in my B23E for 12months now because I found that Mobil 1 wasn't quite thick enough when hot to maintain full pressure (GP50 = 25-50w, Mobil 1 = 5-50w) and I didn't need a cold temp oil because it never gets cold enough here either (Sydney, Australia)

As Paul mentioned, 20W50 is too thick and has too wide a viscosity spread.

10W40 has the same problem with VI.

Why not try a nice 15W40? Shell Rotella and Chevron Delo/Delvac are good choices. Because you are maintaining multiple bricks, it's good to buy the handy gallon size, and that gallon jug makes recycling a snap.

I'll readily admit I'm Mr. Amsoil, Dr. Bias. But with your type of driving synthetic oil would not only be best for your engine, it would save you $.

But if you insist, I won't bad mouth dino oil. I'll stick with the facts on dino 20W-50. DON'T USE IT. The viscosity numerical spread is just too much for petroleum oil to handle narurally, so the blenders add viscosity enhancers for the summer number (50), which typically consists of a thickener that can be considered unstable/not long lived at high temps and pressures. By products include nasty sludge and varnish. On the winter number (20) this is acheived with pour point depressants (ie solvents), which rapidly out gas from the heated oil, only accentuating the breakdown and thickening of the oil. Your owners manual makes no mention of 20W-50 for good reason. (PS the above is NOT applicable to good synthetic 20W-50 which can make the spread without gross additives)

Really I'm not trying to sell anything to you - but if you shoot me your snail mail address, I'll send you a neat 8 page description of motor oil classification (SAE and API) with all kinds of good stuff about viscosity, volatility and just good light reading on motor oil. I promise you won't be added to any mailing lists.