Remember that SAE 30 is higher viscosity than today's TCW3 oil.
These hydrostatic lower units are cool when they are working right - when idling at minimum hydro setting, old Champion repair and parts guy, Lyle Swanson, told me you can stop the prop with your finger, but I never had the guts to try it. They don't like water getting in. If the propshaft seal is bad, that is a custom Champion part. Lyle told me back in the 1980s that he had the molding house make up more occasionally until the mold broke. (when Champion folded and he got all the Champion factory parts, he got some molds too!). He had a new mold made - I don't know if that mold still exists or if anyone has access to it anymore. Back in the early 1960s when I was 14, I bought a 4LHD for $35, got it running and ran it on a homemade hydroplane. That motor is long gone, but I bought another one a year ago that is seized. Haven't gotten around to seeing if I can make it run yet. When running the 1st one on the hydro, due to the higher RPM it revved to, Lyle told me to run gas/oil mix at around 10 to 1, (note the CHampion HotRod racing motors ran on a 4 to 1 mix) and I had to go two heat ranges colder on the spark plugs. I dunked it several times.