Without stepping into the debate about which oil is best, and what if any reprocussions may or may not be present with "alternates" to the oil called out in the TMs, there are two things that I feel should be pointed out-
First, the original recommendation in the TM, the GL-1 rated oil is NOT obsolete and is NOT hard to obtain. GL-2 and GL-3 are obsoleted to GL-4, but GL-4 has not yet obsoleted to GL-5. (I won't be surprised if it does, but it'll take another round of updates. GL-1 has not been obsoleted yet. Why? Well I dunno either. But, I do know that most parts stores have GL-1 on the shelf, and if not it's one day away in their warehouse, no "special ordering" charges or hassles, just ask for it. The difficult part is finding a counter person who will not tell you "you don't want that, this is better..... or telling you "we don't have that" when what they really mean is "I don't remember the part number but we have this other pail of oil right here that would be a lot easier for me to sell to you...." If you decide that you want GL-1 you should get GL-1, and you should not be dissuaded by a counter guy who figures that API just "forgot" to make it obsolete. Worst case scenario, you take the brand that your favorite parts store is selling, spend two minutes on Google, and you can take a part number to the parts store. And hey, nothing beats the feeling of telling a parts guy "see--- I told you that you had that...
Second, in the short time I've been at this site, I've seen it debated over and over, what's right, what's "righter than right" because it's an improvement over the TMs original recommendations. I've seen it in lots of posts and again in this one. It gets thrown out every now and again, but always seems to get blown over and goes relatively ignored. The original correct oil for this transmission is NOT what the TMs call for, the TMs call for an "acceptable substitute" to the original, to match the military's need to minimize the different fluids in use. The original manufacturers recommendation is not gear lube, but 50 weight oil. IMHO if you're going to step away from the TMs in search of "the best of the best", then regardless of the conclusion that you come to, perhaps the original manufacturer's recommendation would be a better starting point as you follow the ratings through the years to come up with a modern equivalent of the original oil.
Anyway, just a couple of things to consider while considering this. I've made my choice as have most of you, and none of you are gonna change my mind any more than I'm going to change yours...
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