20W-50 will be fine to run, no problem at all. On a cold start, it flows like a 20 weight, and it lubricates like a 50-weight when hot. The oil pressure will be slightly lower than it would be if you ran a straight 30, because the physical thickness of the oil is less. 20W-50 does not get thicker as it heats up, but the additives in it allow it to lubricate as if it was heavier when hot. The only real disadvantage to straight 30 would be slow oil pressure buildup due to thick oil on a cold start. Below-freezing temperatures require the thinner 20 weight to keep the bearings from starving for oil before the pressure comes up.
Jerry
