The manufacturer's recommendation for W124 cars is SAE 90.

If I put 140 weight gear oil in my W124, am I causing damage to the differential, because the oil is too thick to penetrate into the tiny gaps it has to?

What are car manufacturers' official positions on using higher-than-specified-weight gear oil in differentials?

If 140 weight gear oil (75W-140, 80W-140) is mixed with 90 weight oil (80W-90, 75W-90) by topping off, will it still provide good protection? Gear oils are designed to be mixing-compatible with other gear oils, but are different weight gear oils compatible too?

Do 140 weight gear oils really suppress differential noise much more than 90 weight gear oils?

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.