Was looking for something cheap to do a flush before putting in the MT90. It is an old MTX pulled from a junk yard.
There are no cheap fluids and there are expensive transmissions. But, I would definitely rinse the rust/water/sand/rodentpeepoop out of a salvage yard transmission, and replace the axle seals or front/rear seals too...
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Even the conventional GL4 from Driven, BradPenn, Stalube, if you can find it, is $15 a quart. So, $20 for Redline/Amsoil isn't much of a difference for the typical 2-3 quart manual capacity.
The Pennzoil GL4 is all but impossible to find. Think I saw it once on a shelf 20 years ago.
For a 50 mile rinse, I would just use any marine GL4 or non-LS GL5 gear oil. If it were my transmission, I would rinse it with Redline since that is the additive package I want in the transmission, and not any previous generic fluids. BTW, I've flushed with 10w40 and 20w50 conventional motorcycle oils(hint hint), non-LS GL5 gear oils, and even HDEO 15w40 too. So, I'd grab a jug of supertech 15w40, fill that transmission, drive it for 50 miles and then drain it out for Redline. Make sure you use a clean drain pan and inspect for rust/metal/sand.... If there is any, I would repeat with motoroil or generic gear oil, or might just be best to return the transmission if you're not capable of replacing all the bearings/synchros yourself.
And, always state the year/make/model/...that you're working on.
I would not use ANY LS GL5 gear oil, 40-50 grade motor oils, or OTR MT1 trucker gear oil, in a passenger vehicle transmission that has GL4 as a recommendation. I would not want any GL5 LS additives either. I don't care if the automaker says GL4 or GL5 or if the gear oil blender say GL4 and GL5. Gear oil manufacturers and automakers make some bad recommendations.