Going through the 120K service myself rather than biting the bullet and never knowing what was actually done and what wasn't

I was under the car (1989 300E) changing the fuel filter last night and noticed that access to the drain/fill plugs on the differential are easy to access (much easier than those on the gearbox of my 94 Carerra!). No where in the manual does it prescribe changing the diff juice. Any thoughts on this? If so, which fluid do you recommend and what size are those allen heads in the drain/fill bolts?

Thanks!