20w-50 havoline in the '78 F250 with 400. 15w40 Delo in the Cutlass with '69 350. 10w30 Havoline in '00 Grand Prix. Older engines have larger clearances, so I prefer a 15w40 or 10w40 in an older engine that's freshly built or a 20w50 with an older engine with more miles or with a small leak or if they smoke some.
We all can't afford $10/qt oil Chris . I have never had any problems with havoline oil & there are several good reviews on it out there. With the looser bottom ends on the 351m/400 engines, I feel a that 10-40 or 20-50 is the way to go. (Winter & summer or climate). 10-40 is good to 0 degrees. I was just throwing that out there on the worn engines, but I'd rather use a thicker oil on a smoking engine or one with low oil psi to prolong its life until you save up the $ to rebuild it or drop another in as to running 5-20 with a smoke cloud behind or no oil psi Lol. I like the 15w40 Delo mainly because it has more zinc for the cams on my older engines. I could use the rotella 10-30, but it's said to have less zinc now, but it's out there.
pennzoil 5w30 conventional, usually change every 3-4 months, milage varies depending on the vehicle, in the blazer its probly about 2,000 miles. (this engine is a junkyard replacement, motor has 200k plus, chassis has 188k) the ranger its about 3,000 to 5,000 miles. ('93 4.0 ohv, has 163k) daughters suzuki reno is probly about a 4,500-6,000 interval (this 4cyl has about 162k) i ususally run purolater filters, used to run ac delco in the blazer and motorcraft in the ford, but heard that the gm filters sold at walmart were cheaper made, so i stopped getting filters at walmart, the engine in the blazer was changed last spring after a oil-system failure, had just had a oil change the week before, related? not sure, but it spun every rod bearing and the middle 2 mains, sounded like a diesel, no power and overheated after it stopped running on me, im surprized it restarted. i had to walk a mile home to get a vehicle to jump it after it just quit on me. the junkyard engine got a new high-volume oil pump and new crankshaft seals (and a good general going over) before it was dropped in the engine bay, so far, so good. 5w30 is what is on the oil fill caps on all 3, so thats what i use, the suzuki uses a little oil, the ranger drops about a quart every 3,000 (and much of that is due to a leaky valve cover on the left bank) and the blazer is down about a 1/2 qt at 2,000 miles.
Years ago, oil was used somewhat as a "shock absorber" between the crankshaft and bearing surface. Because of that, heavier oils were used. Today, everything is machined at such close tolerances, heavy oil is not necessary. We've actually had vehicles that could develop driveability issues if too heavy oil was used. Orifices are smaller, and heavier oils won't flow through them as well.
Should be listed in owners manual. I could try to look up with year and what model and engine size