While most responsible Mercedes owners remember to change their engine oil reasonably frequently, not everyone remembers to look after the other lubricants on the car. This article focuses on arguably the most severely neglected fluid: the gear oil in the rear differential.
Most Mercedes 126 cars have "open" differentials, though a few of the later models came with limited-slip differentials. The former do not require much attention and are, in fact, extremely robust. Hot-rodders have found that the rear differential on the 380SE can handle as much as 800hp through the rear wheels. But, that does not mean we can forget about it completely. The gear oil should be changed at roughly 100,000-mile intervals, or every 50,000 in severe service. If left in too long, it will eventually lose its ability to lubricate and starts to resemble frothy pea soup! (It does not smell too good, either!)
Fluid capacity is only about 1.5 quarts, so there's no excuse not to use the "good stuff." Amsoil, Redline, and Royal Purple all make high-quality synthetic gear oils, typically with 75W-90 viscosity. Just as with an oil change, this is best done with the fluid at operating temperature. The two plugs in the differential – upper fill and lower drain – take a 14mm hex key and are usually extremely hard to budge. A long breaker bar will be required. Make sure that you loosen the filler plug first, because if you remove the drain plug and then find the filler is stuck, you've got a bit of a problem. Melting an ice cube in the filler plug's 14mm-fitting may help loosen it through thermal contracting.
Refilling is much easier with a small oil pump. Simply fill until the gear oil is level with the filler hole; when it starts to see out of the threads you know you're done. To ensure an accurate fill, the car should be level during this operation, though this is not critical. If you leave the rear of the car up on jack stands, a slight overfill will ensue. Torque the plugs to 50Nm (37 lbs / ft).