1968 Nova, 250 six cylinder, 3 on the tree, 2.73 axil ratio, 155HP, max torque@1600 RPM, bought new from Joe Moss in Albemarle, NC. Joe was 60 and lived with his mother. He said to run the factory oil for 5000 miles then use straight 20 weight oil. Ran 20W Kendell for first 150,000 miles. Used a quart every 1,200 miles. Put in a Frantz oil filter at 110,000 in 1972. By 150,000 miles, 20W was harder to get, and the store did not have 10W-30, so wound up with 10W-40. When the engine was hot, it ran like it was full of glue. Hummm... Lower fuel mileage, too. I went to an oil supplier and secured some straight 10W. Ran that to 250K miles and about 25 year old car at that point. The straight 10W oil increased my 10,000 mile average MPG by 1.9 MPG to 25.9 MPG. Blow-by and oil consumption started up. Got careless about oil changes. At 325,000 and 30 years, rebuilt the engine myself. About half the compression rings were stuck. All the oil control rings were caked in carbon. The oil control ring drain holes in the pistons were packed with carbon and could not be pushed out by hand and not that easy to drill out. Took 0.040" to clean up the cylinders. Crank miked new. Rod and main bearings had more corrosion damage than wear. She has 361,000 on the clock, now, but the new cam has a 2400 RPM peak torque and is harder to get a load rolling than the old cam. I get whatever 5W-20, yes 20, name brand oil is on sale and PureONE filters. The Frantz by-pass filter gets whatever 2-ply tissue we are running in the john. And, two ply makes a difference!
Note to self: If a car is in regular use, change the oil and filters (I have 2, remember) every 3 months and pretty much ignore the miles up to 6000 miles. The 3 months is especially important if there are a lot of short trips. If you live long enough, you will wear out the engine. Hopefully, you will be throwing away some good oil, but not throwing away a bad engine. Don't buy into extended oil changes because of a bypass oil filter. If you do, your crank bearings will hold ok, but your oil chemistry will not keep the piston rings from sticking. When blow-by develops, oil is blown off the cylinder walls and cylinder wear increases. The blow-by will also dirty the oil quickly. Rebuild the engine at that point or drive it into the ground. Plain steel piston rings wear the cylinders much more quickly than chrom-moly.
I tried 12 month drain intervals with M1 and was very disappointed when all the rod bearings and the crank failed after 18 months. Did not like the tough, gummy varnish in the engine, either. Also lost rear wheel bearings on two rear wheel drive cars running M1 gear lube. I think M1 was having troubles then that they don't have now.
Driving a car 300,000 miles in 10 years is a lot easier to do than driving a car 300,000 miles in 30 years. Want to do major damage to your engine? Just go out and cold start it. I have read that when the engine is cold, half the wear of a 500 mile trip occurs in the first 10 seconds after and engine is started.
Tale: Widow calls my buddy's garage and says her car won't start. They tow it in and find the 350 GM engine stuck. They opened it up and noted an oil failure. They called the widow and ask about oil changes. Her husband bought the car new and died. She had never checked or added oil. Car ran 85K miles on the factory fill.
So, boys, somewhere between what I do and what the widow did is what you should do.
You don't really want to know about my 17 years with Olds diesels, do you, ...and why 2-ply tissues make a difference? Wore out an engine hoist...