I've been a big fan and user of Amsoil for 20 years in Amphi....but only in the engine and grease (all over). I used it a short time early on in the transmission, but seemed to have a worse problem with migration of oil between transmissions....so I went to a conventional oil years ago and want to stay with it.
With conventional oils being harder to find, where are folks getting their oil? - those who do not go with a synthetic.
I'm thinking the PennGrade GL-4 80-90W
Marc - Summit Racing and Jegs have the Penn Grade GL4. I picked up some GL4,"Sta-Lube" brand made by CRC from NAPA last week. We have a 40+ car collection (not Amphicar, just mine stashed at home) and the old 4 speeds don't like the GL5 or synthetics of any kind! It may not be too noticeable in Amphicars but the "new" oils don't let the old synchros do their job.
For what it's worth, I've run Amsoil 80-90 with up to 50% Lucas oil stabilizer (not their synthetic type since it wasn't available) for 15 years now with no sychro problems at all. That's 17 of the 22 thousand miles on one car with the transmission never being torn down other than to replace seals when I restored the cars. Both cars have smooth, silent shifts.
Most of the 4 speed transmissions we have here get shifted rather violently from time to time (1960's & 70's) ! The newer "slick" oils don't let the synchros work as they should which leads to grinding gears and missed shifts. I hate missed shifts...not much problem with Amphicars! In my experience, Lucas (oil) makes some great products,as does Amsoil.