What lubricant for the "differential" on the mower deck of a GT 3100
 

I recently bought a 2005 GT 3100 with a 54" shaft driven deck. There is a differential on the deck that runs off of the driveshaft and turns a pulley. I pulled the cover off and it has some fairly thick lubricant in it. It is probably gear lube, but I thought I should check here to make sure before I refill it.

I assume you just fill it till it runs out the "fill port"?

How often should this lube be checked/changed?

Check your owners manual and or the service manual for your machine. I wish I had the answer but the newer tractor manuals are not in our data base yet. Hope someone else can help.

Actually, it's a gearbox and you need to pull the cover to drain it unless they changed the design. You will need to stand the deck up and remove the cover and suck the old oil out. Here is what they look like inside.
Attachment 56469

This is what I used for oil. I filled it up to the bolts that come through the case when I had it on my bench but just make sure when it is mounted that the oil is up to the plug on the cover. That plug should be on the top side of the box. I have heard of some being mounted near the bottom but that is wrong.
Attachment 56470

I just made a new gasket and have had no problems with leaks. I don't think the manual tells you anything about when to change or what to use. This box had over 300 hours on it and the oil looked okay.