Time to change the lower gear lube in both of my 1986 yamaha 150 outboards. Can't remember how many quarts i need to do both outboards. Also was gonna use yamalube gear case lube 80w-90 anything better?
For my 150 the LU takes a bit more than a quart, something like 1.1 qts, an annoying value that forces you to buy 3 bottles of oil. I wouldn't use synthetic, the Yamalube is fine. Don't forget to get new gaskets for the oil plugs.
Some synthetics should NOT be used - or more specifically, any GL5 rated lube should not be used in certain lower units. The Merc synthetic is GL4 and is a fantastic gear lube. It's only a buck or two more per quart - it really is good stuff. I've been using this stuff in pretty much everything for a couple decades. Not that the regular Yamaha or even another manufacturer's regular stuff is bad - but the Merc stuff is a step up in that if you don get some water in there, it will still protect the gears. The regular stuff will not fair as well.
I'll second the recommendation for the Mercury High Performance (not their Premium) gear lube. I used that stuff in my old Honda BF90 and the original lower unit is still running fine after 21 years. That span includes a couple of leaky seals and few propeller shreddings. It's GL4 to protect soft metals and it seems to hold up well during water leaks until things can get repaired.