Reading is fundamental and I recommend you look at some of the circle track or stock car magazines. Those guys pay a lot of attention to oil and oiling systems. Most of your local stock car guys need that motor to last the whole racing season. Great information to be found there. I have kept some issues just for references.
Those numbers on the bottle, like 10, 20, 30, are all there because they are federally required flow ratings. In a multigrade oil, the first rating is the cold flow and the second is the hot flow. So a 5W50 starts as a 5 weight oil and a 20W50 starts as a 20 weight oil. And both are rated to flow like a 50 weight when hot. And along with that you should find the "W" oils meet federal standards for cold flow rates or winter use. Before some on this board were born, we would run 30 weight in the summer and 20 weight in the winter. It was a while before some people trusted those multiweight oils.
Castrol GTX has been my standard oil for about 20 years because I like how clean it keeps my motors. My 87T has over 200K on it and it honestly looks like the original tubo. I run 20W50 in the summer and 10W40 in the winter. Lisa drives a 93 Regal V6 which now has over 160K on it. She can tell when I change the oil weights because she sees a drop in hot idle and cruise oil pressures on the guage when I make the change to the lighter oil.
I knew a guy who built motors for one of the major oil/fuel testing labs. In his personal 350 Chevy truck, Corvette, and motorcycles, he ran regular 10W40 Valvoline. In the gear boxes he ran MobileOne gear lubes. And as a neat side note, the Buick V6 was the reference motor for testing in the labs for almost 20 years.
Good Luck,
