Your front diff takes GLS49 (synthetic 75w-85/75w-90), rear diff takes GLS48 (synthetic 75w-140 hypoid, for LSD). Some claim to have luck with M1 75w-90 in both diffs, with a sprinkle of LSD additive in the rear diff. I personally would stick to MB recommendations and stay away from marine gear oil. If all gear oil was the same, then I doubt that Royal Purple and other manufacturers would "waste" their time and money to make dedicated Marine gear oil. There must be some differences in the additives between marine and automotive formulations, that may not cause an issue at first, but will cause a premature diff failure in the long run.