Gear Oil Confusion

[Noah]

Most of the mileage on my 02 V8 2WD Extracab with the limited slip differential is towing a 26' 4500 lb travel trailer so my original plas was to change the transmission and differential fluid every 15K. The owner's manual calls for straight 90 weight gear oil, API GL-5, down to 0 farenheit, 80/90 for colder. I was going to use synthetic, but fortunately, saw the posts on this forum about the seal issues on pre-03 Tundras. Now comes the confusion:
1) No one seems to sell a straight 90 weight gear oil. 80/90 seems to be the closest widely available product. I called the local dealer's parts department and was told they only get gear oil in 50 gallon drums. I then asked their service department what weight they use in the LSDs and was told 75/90 with a top end treatment for the limited slip. They didn't even know that the manual calls for straight 90 weight.
2) All of the local auto parts stores carry a 80/90 that requries an additional treatment for the limited slip.
3) Wal-Mart has a Penzzoil 80/90 API GL-5 dino gear oil that is formulated for LSDs, no top treatment required or recomended.

I went with the Penzzoil and plan to change it every other oil change since it is a) a lighter oil, b) cheap, c) easy.

Has anyone else had this problem? Do you know what the dealer is putting in your differential? Does anyone have a better solution? Also, why is the Tundra differential rated for a 15K oil change and the Seqoia rated at only 5K? The dealer said it was because the Seqoia is heavier. Based on that logic I should be changing my differential oil every 2500 with the engine oil (which sounds nuts to me.)

That said, the Tundra has proved to be an excellent tow vehicle. I added AirLift bags with a compressor and a sway bar. Combined with a good weight distributing hitch and a Prodigy brake controller it easily pulls our camper at highway speeds (never in OD, I'll trade mileage for longevity.) Even for those that don't tow, the air bags really smooth out the ride.

Thanks for suffering through a long post. I appreciate any answers.

[SupDawg]

I think what the owner's manual implies (even though it is vague) is that you CAN use a straight 90 down to 0 deg F. And below 0 deg F. you MUST use an 80W-90. (rear diffs here)
The "W" in 80W-90 means that the lube has been blended (with viscosity index improvers) - and tested to meet a required viscosity at low temperature. The "90" is the high temperature operating viscosity and will be roughly the same for both lubes.
A "straight" 90 may be much "thicker" (higher viscosity) at sub-zero temps.
So - for everyday - all year use 80W-90 is the lube of choice.
About the only place you will find "straight" 90 any more - is truck stops that service semis and BIG trucks.
The 75W-90 is further blended with the VI improvers to meet a lower viscosity yet. The down side to the highly VI improved oils is that under very high loads and temps the VI improvers may "break down" and shear into smaller molecules, hence losing viscosity.
So - the most stable lube for high load - high temp IS the "straight" stuff.
I hope this adds to the confusion.

[Noah]

Absolutely. Now I will never figure it out.

Actually I was familiar with the oil rating scheme. My concern is that since I primarily use the Tundra as a tow vehicle, I might shorten the life of the differential by using the lighter oil. I was surprised that Toyota didn't even know what is in thier manual, and doesn't seem to care. That said, I have yet to hear of a rear differential failing unless it was run without lubrication, thus the 80/90 will probably be fine. I did wonder what other Tundra owners are doing, but the post didn't generate much interest (probably too wordy.)