Can someone explain in plain terms what the difference is in these weights of gear oil. Is the post weight the high temperature rating and the pre weight the low temperature or what gives? ??????

I had my gears installed and the kit I purchased came with Red Line 75W90. I noticed yesterday a howling noise when I turn a corner with some speed. I wonder if this is due to the oil difference? The gears were installed at the Local mod friendly dealer by their master technician who told me that he didn't think the difference in gear oil would make a difference as long as it had the friction moddifier additive.

 

The second half of each number, either the 90 or 140, refers to the viscosity at 210 F. A 90 is between 14 cSt (centiStokes) and 25 cSt and a 140 is between 25 cSt and 43 cSt at 210 F.
The first number refers to the maximum temperature at which the gear oil will reach 150,000 cP (centipoise).
A 75W will go down to -40 before it reaches the 150,000 cP.
Therefore, the 75W-140 will be inherently thicker (heavier) near your operating temperature, even though your operating temp is probably closer to 100 F than 210 F.
Please remember that the ranges above can also be pretty close to each other. (In other words, you could have heavy 90 that is almost the same as a light 140.) I can't speak to the Redline, but they typically make good synthetic products. It may be that your differential needs the hydrodynamic viscosity that the 140 offers.
Good luck.

 

So in other words the howling could be caused by the gear lube getting too hot because of the 90 rather than using the 140? I hope this is the answer I hate to think it could be somthing else. READ: WORSE

 

I guess i'll try a lube change before I lose my cool. I hope that it fixes the situation.