Should the SAE 80w gear oil I use in my tractor be ok to use in my Honda Foreman 450s gear case and defferential? I have plenty of it and would like to avoid having to buy more is what I have is sufficient.

 

The tranny fluid is the same oil as the engine, same sump, its just motor oil..
The differentials use gl-4, so if thats what your tractor has, then its the right stuff. The service manual says just to use hypoid gear oil #80 weight.
The older foremans, with the limited slip clutch in the differential also had some friction modifiers added, i believe they were pre 98 and older...what year is your foreman?

 

Yes it will be fine, I usually run synthetic in my differentials

 

I find it scary that you used the term GEAR CASE! The engine and transmission on your Foreman SHARE THE SAME OIL, and there is NO seperate "gear case" as such on your machine that takes any type of gear or transmission oil. I hope the fact that you don't know this indicates that this is a new machine for you, and not that you have never changed your engine/transmission oil in all these years!

The only ATV that currently seperates the engine and transmission oil is the Honda TRX450R sport bike........God Bless Honda! Use an API SG/JASO MA motorcycle specific oil in your engine/transmission. (No way do you want to put 80 weight gear oil in here!) Consult your owners manual for the weight that is appropriate for your climate and season of the year. My personal preference here is Maxima Extra, ester synthetic.

The differentials is the only place where you will be running "hypoid gear oil". My personal preference here is Mobil 1, 75W90 synthetic, which is GL5. (But I'm however probably going to switch to RedLine ester synthetic next oil change). The diffs hold such a tiny amount of oil, that the expense of running an expensive synthetic is virtually nothing. (You can run a GL5 oil in a diff that requires GL4, but not GL4 oil in one that requires GL5.)