I had one of my lower drive unit replaced (Volvo Penta 5.7 GSI with cobra SX drives) and instead of the shop putting in the Synthetic GL-5 75-90W drive oil in they swore that the mercruiser synthetic GL-4 90W drive oil was a better performance oil. It's a blueish color compared to the clear Volvo Penta 75-90W oil. Now I have twins with one drive using the Volvo Penta GL-5 75-90 (clear) and the other drive with the blue GL-4 90W. Should I be concerned? Thanks for the feedback!!