Classic dino oil as well for the 79, castrol 20W50, with less than 1000 miles a year. I tried the synthetic on the 85 Honda and it started to leak like a sieve, reverted to classic dino and the leaks have stopped...
On the jeep rotella (diesel specs) 15W40 comes in inexpensive gallon jug at wallyworld for $13, goes in the bimmer too for the last 10 years

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Michel
74 wagoneer
79 450sl