You need a quart of gear oil for the rear diff and a quart of gear oil for the transfer case. The front diff shares ATF fluid with the tranny, that's why you have two plugs to remove when you do the tranny fluid.
Carguy, you are great and u rock:, wow I had so much of confusion and was not sure as i had purchased 4 qts of Royal Purple 75W90 thinking I would need 2 for the front differential & Transfer case and 2 for Rear differential and Transfer Case, but with your response it was clear, I just did a rear differential oil change, i still need to do the front transfer case, but not able to get the right tool to open the filler bolt as there is no place to even place a 24mm box wrench. Do you know what is the best tool in that spot and what size i may need to use. I have a 2000 RX300 AWD with 102K miles on it. and of course the major problem is oil leak from rear main seal. just dragging along with it, as it is too expensive to repair the main seal.
Thankyou once again for the right direction.
I just use a 1/2 socket wrench with maybe 10" in extentions to get to it. Use either 15/16" or 24mm for the socket (they are almost exactly the same size). The fill plug has a very thin head, so you need to push on it hard to keep the socket from jumping off. The drain plug can be tough too. There is not much room to get a wrench on it. I took someone's advice on here and got a 15/16 wrench from Ace Hardware it's their "Pro Series" to do the transfer case drain. It has a very thin walled box end and fit the plug with no grinding (worked on by brother-in law's RX330 too). http://www.clublexus.com/forums/5090318-post9.html
Front Dif uses Trans fluid because it is tied into the transaxle. (change every 15-20K miles along with trans)
Transfer case uses gear oil and is located just aft of the front differential/transaxle. You can use 80w90, although many guys on this forum like the 75w90 Synthetic. (change every 25-30K miles)
Rear diff uses the same gear lube as the transfer case. 80w90 Dino or 75w90 Synthetic (change interval is the same as the Transfer case 25-30K miles)
The intervals are an average of what I've read other members on CL are doing. Synthetic can go on a longer interval, depending on driving conditions.
I changed my rear differential oil last month and it was last done at the dealership before I bought the car, about 25K miles ago. It looked new. I'll be doing the transfer case oil in the next couple weeks, and I'm assuming that it too, will look new still....