Join Date: Mar 2003
Location: Fairbanks, Alaska
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Re: Fluid in a T5 tranny
I can not say what fluid is best in a T-5 however, I can offer some thoughts on the transmission itself as well as what is required of a lubricant in a transmission.
I have Destroyed 2 T-5's. One World class (WC) and one Non-world Class (NWC). Both failed in reverse. Both resulted from dropping the clutch at 1500 rpm on a stock 258. In both cases the failure started with a mild release of the clutch in reverse, that lead to a noise when in reverse. Later when under load the reverse idler failed. Both transmissions were running ATF at the time. The ATF definitely helped with shifting in the cold. Both failures Where due to only 2 components, thous were the reverse idler gear and first/reverse gear on the cluster gear/counter shaft. In both cases the teeth on the counter shaft showed signs of cracking under load and deformation to the teeth, and a complete failure of the reverse idler. These would be caused by excessive load and not by improper lubricant. When the Transmissions Failed the Teeth were broken clean off the idler gear, this would point to an improper hardening of the idler gear or a very excessive load. In the case of the NWC T-5 this lead to the complete destruction of the reverse/first gear on the counter shaft and the destruction of a reverse idler. In the case of the WC T-5 this lead to Cracks in the reverse/first gear of the counter shaft, and the destruction of the reverse idler.
In both cases the bearings and gear tooth surfaces looked good. It was the material of the gears themselves that failed. When it comes to the strength or lack there of, of the T-5 I feel that the lubrication has almost no effect. The real issue is weather it is strong enough to stand up to the max torque of your engine. Do not be fooled by claims of the HP this transmission will stand up to. HP destroys things with heat that leads to lack of lubrication. Making choice of lubricant very impotent. However Torque destroys things with load, this makes lubricant less important at low RPM's. However the gear must stand up to the torque the engine can produce. If I could do things over again I would swap in a np435, T18, T19, SM420 or SM465 rather then replace the T-5. If your T-5 is okay and you mean to keep using it then worry more about over loading it, especially in reverse, then about your choice of lubricant.