I'm going to chime in here, not because I know a darn thing about oils (I learned a bit in this thread, thanks slo-mo) but because I owned and ran a 1973 Gulfstar with twin Perkins 4-236 engines for 8 years. We put about 1,600 hours on it, and a lot of miles.
Almost every 36 Gulfstar with twins that I have seen (assuming original power) had 4-236s in it, so I thinks it's pretty safe assumption that those are the engines in his boat.
1. The motors won't turn 3,400 rpm if the tach is accurate. WOT as I recall was about 3,000.
2. They love to run at about 1,800 rpm. Much above 2,000 and the boat just starts to squat and doesn't really gain much speed while burning more fuel. Possibly more oil as well?
3. 1,800 RPM in a 36 gulfstar with twin 4-236, both engines running, with a reasonably clean bottom and props results in a speed over ground between 7 and 8 knots, assuming no strong current etc.
For the purpose of getting the boat home I would run it right around 7 knots or so on the GPS, maybe a bit less. I'd feel comfortable that no harm would be done to the motors at that speed and you'd be running close to the max efficiency for your boat.
After I got it home I'd get the tachs figured out. Try running new grounds to the tachs, pull the grounds off of a clean source not the other tach, maybe even add a new buss behind the dash. I had to do that on all four of the tachs on my boat.
Hope that helps!
Doug