All of the drain & fill plugs take a Hex head socket (Allen head). Front Diff takes 1.30ltr, the center is .85ltr, and the rear is the one I'm uncertain about but I believe it is about 2ltrs so 3ltrs total should be enough to do all 3 but if you have 4 on hand your guaranteed to have enough. The front Diff has no drain plug, just a fill plug so the fluid has to be evacuated from the fill plug which is just to the front side (about 2 to 3 O'clock position) on the right side front axle flange. The center diff has a drain plug on the bottom rear of the transmission tailshaft housing about 10cm or so from the axle and the fill plug is on the right side about 1/3rd of the way up the housing above the drain plug. The rear Diff has the drain plug on the bottom and the fill plug I can't remember if it is on the right side (pretty sure), or on the left side. You do have to remove the sheet metal splash pans around the rear diff to get at the fill plug. The fluid is definitely 75w90. The exact fluid recommendation is called: Burmah SAF-AG4 for the front & rear and Burmah Carbon Mod. for the Center Diff (or the SAF-AG4 w/.034 of friction modifier STURACO FM 1992 added). A high quality 75w90 will do the trick although I am partial to Redline as their oils all seem to exceed all oil specs by more than other synthetics. I don't know why they specify the Burmah w/friction modifier for a Torsen diff (knows as a Parallel Axis Torsen) but they do. I have run a couple of Torsen Audi's on regular 75w90 for years (Redline 75w90) with no problems but the closest Redline for the Center Diff would be 75w90NS but when I bought my car in '05 I changed all Diffs with regular Redline, it wasn't until I just reinstalled my engine/tranny last month and refilled the tranny did I actually use the 75w90NS in the center diff and can't tell the difference. And yes all Diff fluids are listed as "fill for life" and only spec'd to be checked & topped off per the shop manual (B.S. is what most people say). Just like the fill for life transmission fluid when even the manufacturer (ZF) specs a fluid change interval every 60K~80K in general for the 5HP-24A tranny whereas I feel the tranny not being beefed up we know enough for the RS6 can use oil changes more frequently so I do my ATF every 40K & Diffs about the same although the Diffs are probably fine going a little longer.