To be honest, I'm not all that sanguine about their advice, either. They recommended 75W-90 for the E36 M3's differential for a long time. The OE fluid was classed as a 75W-90 before the gear oil viscosity grading system was updated in 2003; after that, it would have fallen into the 75W-110 category. BMW back-specced their 75W-140 for that car; if anyone would know better than Red Line, I'd imagine BMW would. Red Line stuck with 75W-90, which is considerably thinner than the spec fluid. I never heard any explanation other than "we've run it in these diffs for a long time and never had a problem." Which, I'm sure I don't have to tell anyone here, is probably the most meaningless statement in the business.
That said, it couldn't hurt to ask, right?
That said, it couldn't hurt to ask, right?