Splashoil, we have a Hydrovac on our Diamond T and also wondered what type of oil to use. It is recommended that you place 1 oz. of oil in the cylinder plug every season or 10,000 miles. Our parts man told us to use Marvel's mystery oil. He said that is what they always used. Sounded kind of strange but he has been in the business since 1952 and has been on the mark on every other question we have had for him. Anyway, we followed his advice about six months ago and so far the Hydrovac is working fine. The current Hemmings has ads for rebuilds (July, pg 4384). Good luck.
