best type of oil for 71 340 engine

The reason I'm question oil viscosity , about ever 2 yrs I strip the engine down of all of it's accessories , hoses, wiring , radiator , etc and do a repaint and cleaning of the engine for car show season , I've noticed that there is a slight seepage of oil from were the heads meet the block , I was courious if my oil was too thin and if it should be thicker for start up porpose