If your '86 has the factory engine, it has flat tappet lifters. The SM-SN oils also have a rating called "ILSAC GF-4" or "GF-5". To get that rating, you have to limit the amount of "ZDDP" (short for "zinc dialkyldithiophosphate"), an anti-friction additive that is critical for high-load shear applications. Which is exactly what you have with flat tappet lifters. Any engine that requires an SM or SN oil will have roller lifters, so the low ZDDP isn't an issue.

You should use an oil that has a SL/CI-4+ rating. The "CI" is a diesel rating ("C" = "compression", as in "compression ignition"), and the "SL" means it is also rated for a gasoline engine ("S" = "spark", as in "spark ignition"). You don't want to use a CJ-4 rated diesel oil, because it also will have reduced ZDDP.

Sorry, romance, you are dead wrong about oil bases not breaking down. While the VI improver additives are usually the first motor oil constituent to break down, a cheap base will be the next.

These "what oil" threads usually turn into flame wars, because of biases and misinformation (usually un-information). Hope that doesn't turn out to be the case here, but I'm not going to hold my breath.

Судячи з американських форумів, доля цинку для старих валів повинна складати 1200 часток на мільйон,

...сучасні масла містять 600-900..

Значить, в 2-х словах:
хто юзає ... на flat tappet валах, той додає один флакон на 5 літрів

...масла з маркуванням SM-SN (більшість сучасних)...