They spec synthetic because with a good synthetic oil it's likely to make it out of warranty with no service whatsoever. That's the only reason.

 

I'm not too sure the AAMs came from the factory filled with synthetic. Chrysler says to use their gear oil or equilvilent expecting most just to let the dealer service them. Synthectics are more stable and work well in cold climates but in the south there's not much need for synthetics. I would rather use cheaper oil and change it more often. I have seen some synthetics that weren't blended properly disolve seals. Synthetics have to have a additive to protect the seals from disolving. Other than that synthetics have the same additives as dino. To be classed as GL-5 the oil has to have the same additives that mostly come from the same place. Anything differant it's not GL-5 so all GL-5 oils are just about the same.
Billy

 

Exactly. My manual doesnt say you have to put synthetic in it. It says GL-5 synthetic or equivalent. I opted for the much cheaper equivalent.

 

GL5 is an API extreme pressure lubricant performance rating. Synthetics of the same rating are not "equivalent" to their conventional counterparts in every sense of the word. Synthetics have much better performance in certain other aspects of service like resistance to shear breakdown and cold weather flow properties.

They are only "equivalent" in that sense of the word at room temperature the minute you open the bottle. After that, all bets are off. I would be more inclined to rely on sythetic in the diff under hot conditions as opposed to cold, simply for the shear resistance and nothing else.

 

After my rear end started chatering with AMsoil, I switched to RP which helped some, but since it had excess wear show in the sample I sent to Blackstone, I swiched to din gear lube with th trans-go posi additive. I went with the 85-140 Valvoline and have had no issues since. Even towing heavy.
Mine is out of war, so it doesn't matter.

 

GL-5 is shear stable for hypoid gears. Dino or synthectic.
Billy

 

Sure. But the synthetic is "more" shear stable.