When my ordered '77 Camaro came in in 05/77, I did the first oil change at 3000 miles. Went with Castrol GTX 20W-50 and oil consumption was 1/2 qt/4000 miles. Somewhere along the way, like at 250K miles, it was 1qt/4000 miles. As things progressed, the oil was still pretty clear, so I would put a quart in it and run it until it was 1 qt low again before I'd change it. At that time, I was doing my own oil/filter changes. Upgraded to Castrol Semi-Syn 10W-40 later on, then to Rotella T 5W-40.
When we finally put in a pre-built 355, the intial oil was the engine installer's choice, Valvoline 30W. After about 3000 miles, I changed it back to Castrol 20W-50. I immediately noticed that throttle response and pwoer were just a bit down, so I changed it to 10W-40 and those things came back like they used to be with the 30 oil in it. Lesson? As reported, the thicker viscosity oils do consume more power, just a tad, but enough for me to notice. Later, went to Rotella syn 5W-40 for the zddp content.
Ended up putting 625K on that old 305. Pulled it out as all of the block freeze plugs were seeping. Figured that IF I could get a Chevy motor to last that long, a MOPAR would be forever. One "assist" was the Cloyes Plus-Roller timing chain at 92K miles.
As things have progressed, a quality rebuild with quality machine work at or near min-spec clearances, OEM-quality gaskets, with quality syn motor oil (probably 10W-30 or 5W-40), a good oil filter, and ANY Chrysler V-8 should easily out-last your wildest suspicions of longevity. Proving the integrity of the original design and execution, to me.
Just some thoughts and experiences,
CBODY67