[38Chevy454]

I don't think that is matter of lube causing the breakage. I will get a bit metallurgical tech here:

Aluminum does not have a fatigue limit, so it is quite possible that the bar did develop a crack and it propagated until the final overload failure. If I could see the breakage fracture surface in detail it might confirm this. Could have a mfg defect that enabled a stress concentration, which makes the fatigue crack problem worse. Aluminum is not as good of a choice for a cyclic tensile loading situation. Your 50K miles would represent a lot of cycling.

Overall, I think a steel tow bar is a better choice. Bit heavier and more to wrestle when putting int he receiver, but I doubt the stresses would ever be above the fatigue limit for steel, which does have a limit (approx 50% of yield strength) whereby no fatigue cracks can initiate or propagate.