Why overthink it?
The 3.0 has had SAE 30 weight, 10W-30, 10W-40, and 20w-50 recommended in publications from Mercruiser, Volvo Penta, and OMC, and the motor has not changed a bit since 1991, with only slight changes from the mid 70' to 1990 ant the 2.5 liter that it was derived from had less...