If you want to get more involved with learning about gear oils and what they are required to do in a differential, here is a good starting point:
White Paper - A Study of Automotive Gear Lubes
Although I am not an AMSOIL user, I think their 75W-90 SVG would be a great choice for you in the front diff and xcase, and their 75W-110 SVT in your rear diff. Since we don't have LSD, additives are not required.
For the first 6 years that I have owned my 2000 Tundra, I used to just take it to a dealer for diff and 4X4 servicing every 30K. I never really knew nor cared what they put in the diffs, but I also never had a problem with anything. Then, after re-reading the owners manual again, I decided to start using what Toyota recommends. About the same time, I decided I was going to be the first and last owner of my truck. To make a long story short, I'm now using Valvoline 75W-90 GL5 full synthetic in the front and xcase, and Lubrication Engineers LE 1605 SAE 110 GL5 in the rear (
Lubrication Engineers - 1601-1609 DUOLEC™ Vari-Purpose gear Lubricants). I will probably stay with the LE 1605 in the rear, but switch to either Mobil 1 75W-90 or AMSOIL 75W-90 SVG in the front and Xcase on my next service (I do it myself now).
Why does Toyota recommend 75W-90 in the front and xcase? Remember that "W" stands for "winter grade", not weight. The Society of American Engineers established certain cold weather property tests to establish whether a SAE 90 or 110 can be labled a multi-grade winter oil (75W-90, 80W-90, 75W-110, etc.). If they don't meet this criteria for extreme cold weather, then they have to be labeled SAE 90 or 110 instead of a multi-grade winter gear oil. In order to meet winter multi grade, oil companies add viscosity improvers to make the base oil pass the low and high temeperature viscosity tests. These viscosity improvers, as well as base oils, come in verying degrees of quality and can lose their properties over time in use. The result is shearing or thickening out of grade. Since our front differential and xcase are not full time, they are only called upon to help us when we are in trouble, or about to get into trouble. Thus, there usually isn't time for the oil to warm up to it's optimum lubricating and extreme pressure handling capability. So a thinner viscosity and lower temperature pour point is important. Since our rear diff is fulltime and handles a greater work load than the front, shearing out of grade can be a major concern. If this happens over time, the diff will run hotter and be able to handle less extreme pressure. These are enemies #1 and #2 for your rear diff.
The bottom line, IMO, is you get what you pay for in a gear oil. If you want to use a multi-vis GL5, buy the best that you can afford. Yes, it is difficult to find SAE 90 or 110 GL5 at a dealership or automotive parts store. You would have better luck trying a motorcycle dealership or marina where these grades are very popular.
Differential Fluid Viscosity
Linear Mode
