Ask the people who you bought it from what they used. It isn't good to switch to a high detergent oil or a synthetic oil in a high mileage vehicle if you haven't been using it right along. It will dissolve deposits and you will probably start leaking oil from the engine gaskets. Whatever the previous owner used, sounds like it was working. In general, get an API spec SG,SH, or SI The higher the better. The manufacturer probably recommended a 5W-30 when the vehicle was new. If the old owner was still using it, continue using it. Heavier oil will generate more heat from fluid friction than an oil of lower viscosity. If the engine is leaking oil from the gaskets there is probably enough wear in the mains, rods, and overhead valve rocker assembly where you can run a heavier weight to slow the leaks down, because you have enough clearance between the moving parts.