r means resistor, c means copper and s means silver.

the oe plug in 83 was the w7ds. silver is exspensive so most people opted for the w7dc. then volvo took the resistor off the plug wires-the metal shield-and went to wr7ds plugs. again, wr7dc plugs are what you get because of the price, usually. i have never run s's in my cars ever. either ngk or bosch coppers. i didn't see the benefit at the dealer during the 80's. other cars, bmw, audi and vw come to mind, are fussy about plugs. the multiprongs seem to do better than the single prongs in those cars.

if you suspect lean, then the amm may be bad, the fuel pressure regulator is bad, or, maybe, the injectors are clogged. saw that once on a volvo, seen it alot on bmws. and use premium, the regular is not the correct octane for the car. read the owners manual. good luck, chuck.