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On an A2 is under the hood...passenger side, back by the firewall.
You are making a valid point. It all depends upon whether or not you want to adhere with the original manufacturer's recommendations. My Allison dealer told me what Allison recommends. If you bought a transmission from them, and put 10W or 15W40 in a new transmission, and it failed, I believe you'd void the warranty. Call them up and ask. I recognize that our transmissions are no longer under warranty, but I don't have the money to buy a replacement (at least I don't want to spend the money). Your point is basically, "if it ain't broke, don't fix it," and you are correct. Our trucks rarely get driven hard and the mileage is limited (usually). Still, I went through the trouble of changing fluids to coincide with Allison's recommendation. I will add that they did not seem REALLY concerned that the transmission had 10 weight oil in it, but they asked me if I wanted that back it, or the recommended fluid.
I think that they wanted parts/fluids commonality. As you can see, and is proven by all of the trucks coming out, other fluids besides Dexron and Transynd work. I have been told that Dexron is nothing more than 10W oil with additives, and I'd think that Transynd is the same except it is synthetic. I suspect that the figured it would work for as long as they needed. Remember, most of these trucks have fewer than 30,000 miles on them. In civilian hands, these same trucks would have 100K, 200K or more miles on them. So, they probably figured that they'd get enough miles out of them. That is the only thing I can figure. I can't cite test figures or research that has been done; I can only tell you what I was told by my local Allison dealer. Call the one closest to you and ask them what they recommend (not what would work). Regarding my last point, I read somewhere that WATER is an emergency substitute for fluid in these transmissions. I can't recall where I read that, but I definitely did and it might have been on this list. I don't expect you'd get many miles out of that transmission, but it might get you out of harm's way.
Well, my info was not my personal opinion and I stated that. I was told that by an Allison dealer in Landover, MD. I was told that 10W oil was fine, but they would RECOMMEND Transynd or, barring that, Dexron. I went with Dexron. I never asked about 10W40. Perhaps, they would have the same response about that as they had about the 10W. As I said in a previous post, they did not seem troubled by the 10W, only that this is NOT what they would recommend. Acceptable and recommended are two different things. Again, maybe they have changed their positions on this. This was a few years back, but it is the most current and trusted info (straight from the dealer) that I have.
Warning: I'm no expert....
I liked your disclosure. I should have used the same thing.
The different oils all fall into different ideal ambient temperature operating ranges with Dexron being the lowest. (excluding the 00w20)
