On Mobil 1 gear oil bottles, it reads "light duty" while on the cheaper 80W-90 and 85W-140 bottles it reads ". . . for heavy duty trucks . . . "

Does this meant that Mobil 1 75W-90 and 75W-140 give less protection than conventional 80W-90 and 85W-140 for heavy duty applications?

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