Hello, LTJ,
I used to have the same car, though it was an '86. A wonderful battleship to drive (thus my screen name), powerful and pretty reasonable on gas considering it weighs 4000 lbs. and is powered by a V-8 designed in the '70s. I ran it to 197,000 before it was totaled by a moron teen girl sideswiping it while it was parked. Much as I love my gold '97 C230, I wish I still had the big grey beast.
In Denver, CO, I used 20W-50 in summer and 10W-30 in winter, usually Valvoline. When I came back to New Orleans, which (dammit!) hardly ever cools off, I switched to Chevron Delo 15W-40.
At 180,000 miles, my oil pan developed a hairline crack and leak. When my regular mechanic replaced it, he told me, the engine internals looked as clean as the proverbial whistle.
-- Paul W.