From what I know, it's not really a synthetic vs dino oil thing. It's the "rating" of the oil, GL4 vs GL5. For example, Merc's Hi-Perf synthetic stuff (excellent oil) is rated GL4.

For what it's worth, I learned the "rating" thing here on this site.

synthetic, non-synthetic your choice BUT you do need to be aware of the GL-4 vs GL-5 rating.

The potential problem is that IF there are brass/bronze parts in your LU,

using lube labeled as meeting GL-5 may damage those "yellow metal" parts.

Really appreciate the responses.
So to be on the safe side use a gl-4.
Again thanks for the help.

My owners manual calls for Hypoid gear oil SAE#90.
What would you use ?

simply use the rated standard 90wmarine gear oil and go play.
there is no other way to extend gearcase life other than keep it changed.

no advantage to GL$ and GL% can and will kill it.
I found this out some years back the hard way.

Thank you very much!!!!

lemmie tell ya a story bout a man named Bill.
I bought old man bill a slightly used 2003 F50 that we sold a customer and took it back on trade a year later.
( we told the guy it was not enough motor)

so it sat from 04 till 07.
I bought it and gave it to Bill.
knowing how Bill(commercial crabber that was in his late 60's) did maint I used Volvo GL5 synthetic in his case thinking I was doing good.

the case started knocking about 18 months into it.

I disassembled the case and the bushing that rides on the shaft for fwd gear had peeled out and the gear simply wobbled on the shaft.

well it was the season so I simply ordered a case paid for it and installed it and Bill was good.

that winter at a class in Kennesaw I asked why do we have to stock Yamaha HD full synthetic
AND Yamaha premium gear lube.

couple days later parks came back with an answer from engineering.

if the case has NO bronze parts you can use GL5.
otherwise GL5 will not properly lube bronze and some of the additives may actually cause the bronze to fail.

soon as I got home I changed Biil's oil back to premium.

trying to do good I caused it to fail.
now I know.
at least it did not cost Bill anything.

and that is how I know using an oil not specified can screw things up.