Use a multigrade detergent oil. The best you can buy. I agree with Tim about using a diesel oil, as they are usually the best. I have been running 10W-30 in mine since 1973. I rebuilt the engine (1953 235 powerglide) around 1984 and have been running Lubrication Engineers 10W-30 since then. No problems at all. Engine still runs like new.
Kurt
