The differential has two plugs, a drain plug and a fill plug. They both need a 14 MM Allen wrench. They could be real tight so use a socket type that goes on a breaker bar. Loosen the top one first, then go for the drain. This way you don't drain the fluid, then find that you could not get the fill plug loose. I believe the lubricant is 80 weight gear oil. Get the stuff with the additive in it for limited slip differentials. The 560 SL has a limited slip. You will need two quarts, as one will not be quite enough. Warning: the old lube will smell bad, don't get it on your clothes or garage floor.