I have a 1988 Toyota Corolla with 5-speed manual transmission with nearly 200K miles. The last time (24K miles ago, 2 years) my mechanic checked the transaxle fluid and rear differential, he said it was "clean" and didn't need replacing. Up until then, I'd only had city & highway driving, going into dirt roads only when necessary (say to turn around), little snow, and no mud or dirt. Since then, I've taken it into more rough environments (rural areas where the roads are all dirt). I also plan to use it for more snow driving this coming winter. The service manual calls for changing out the rear differential every 20K miles or 24 months only when the car is being used under severe conditions (towing a trailer, many stop-and-go trips [taxi], or in mud, snow, and dirt). My confusion lies with both what sort of oil to use for the transmission and rear differential and with the oil draining. First, the shop manual conflicts itself on which which oil to use for the rear differential. In general maintenance section, the shop manual says to use ATF Dexron II, while later in the section on the repair of the rear differential, it says to use hypoid gear oil API GL-5 SAE 90 (above zero degrees F) or SAE 80W-90 or 80W (below zero F). Which is correct? What is hypoid gear oil?I am going to talk to the dealer and to my mechanic. I'd like to do the change myself because I want to put in Amsoil GL-5. Interestingly, according to AMSOIL's site, the rear diff should take SAE75W-90 or SAE 80W-90. So, can and should ATF Dexron II be used in a rear differential? Secondly, the shop manual calls for SAE 75W-90 or SAE 80W-90 GL-5 in the manual transaxle (which includes the transmission, front and center differentials, and transfer). The owner's manual says to use "Toyota transaxle oil E50 or multipurpose gear oil API GL-5 75W-90, 80W-90 or 90" (above zero F) or "75W-90, 80W-90 or 80W" (below zero F.) I am unsure what is in there now and I can't find it in the records. Also, there appears to be several drain plugs for this and I wonder if I have to open and drain all these and fill through each filler hole, or are these connected and can I just use one. The service manual shows one fill and drain hole being used, but there are clearly many more. For the transmission and transaxle, I want to use Amsoil MTG or something similar. However, MTG is rated API GL-4 not GL-5. Will this be a problem? Currently shifting is fairly smooth. There are no leaks from either the transmission nor the rear differential. However, my engine does have a slow front seal leak, so I have been using dino oil in that until I either get it fixed or get another car.