I am having a tough time getting the 75-140 synthetic gear oil, but there are plenty of 85-140 conventional oils available. Does it matter if I use a heavier wieght oil, I tow alot in the summer heat. Thanks in advance.

 

That was funny.

Advance Auto has Mobil 75 -140 synthetic on the shelf here.
Autozone only has Mobil 75 - 90 synthetic on the website.

NAPA online is getting to be a mess finding anything.

Given the heat where you are located I don't think the heavier oil will be a problem.

 

you can use any conventional gear lube sae 90 sae 80w90 etc.

 

i prefer to use an example for emphasis on my point. The main gear box on our 4 row potato digger would get so hot the paint would literally burn with 90-140 conventional. After installing synthetic 90-120 it would still get quite warm but you could keep your hand on it. Now you can draw your own conclussions, hope this is helpful