some years back I talked with an engineer for cummins/truck program that was tasked with working on the getrag package . He informed me that the 5w30 was not sufficient for the 360 getrag . It was done to squeeze all the performance out of the package that they had . They had failures in testing but his superiors felt that there wouldn’t be enough failures in warranty to cause many claims so they left lube as was . It was only after mass production that the failures started mounting up .. this engineer was sent to the vendor to inspect assembly line, where he discovered that they were putting a .045 shim pack in all rags , without checking bearing preload ( this is primary reason so many failed), ( and gave the getrag a bad name) and according to his findings ; this as well as the 5w30 oil and improper bell housing alignment ( caused by poor tolerance cummins adapter plates) , caused about 98 % of all failures. It is also correct 80w90 gear lube is too heavy to properly lubricate .. he said 40 wt is what the rag needed , and upper brass wouldn’t hear of it . That’s all he ever uses in his , and that is what I use in mine without failure one. Amsoil I believe to be superior , but any 40 wt is better than factory spec.