>>>There was a little grease on the gears, but the bulk of it was pooled in the bottom of the case. Temps of -30 to -40F are not uncommon here. It just seemed stiff even in my unheated garage which is warmer than outside temp. I'll see if I can order some L3. Is the Snapper 1704636 worm gear oil similar?
Synthetic 75W-140 GL5 MT-1 hypoid gear oil is -50 capable. L3 is synthetic 75W-140 GL5. The Snapper lub is “cold weather”, synthetic oil and probably 75W-140 GL5 also. Ariens is ok and the Snapper also.
>>>What exactly should the worm gear tooth profile be? Should the top of the tooth be flat or have a slight saddle where the worm drive contacts it?
There is a slight concave run on the top of teeth. The wear is indicated by the thickness of the concave flat on the top of the teeth. The tops should be smooth and unscratched.
>>>It looks like the pin is fine but the thrust collar definitely has scoring on it from rubbing on the now flangeless bushing.
The collar is pinned to the shaft which with good parts has slight front to back movement. I’m not sure about collar contact with the flange surface. The worm is on the top of the gear and pulling forward as it moves the gear so probably no contact between flange and collar under load. You’ve ordered a collar so no issue.
>>Are the collar pins re-usable?
Unless you took a hit I’d re-use it. The pin is 3/16 x 1 ½ so you can pick that up locally.