Info from another online forum:

"Between 1939 and 1958 Gray Marine produced eight different four cylinder marine engines.

All engines used SAE 30 weight oil.

The Gray Marine manual shows two types of water pumps, a fresh water pump with a cast-iron impeller and a raw water pump with a six fin Sherwood neoprene impeller. For the neoprene impeller just compare the size/configuration of your impeller with the outlines shown on page 702-703 of the 2008 West Marine catalog. Any marine supply store will have a similar chart."