I have a 1987 Mercruiser 470 that I normally run straight SAE 40 oil during the boating season. Anyone have experience using SAE 30 in this motor during the summer months? I'm having a hard time finding SAE 40W. Is SAE 30 too light? I don't want to risk any engine damage because of a different oil. Also, the motor has less than 300 hrs. on it, don't know if that changes anything as far as my question goes.