Confused Open differential gear oil

Joined
Feb 19, 2006
Messages
605
Location
Stewartstown PA
Redline 75w90 ns and Amsoil FGL seem to be equivalent to Motul gear 300 75w90 in terms of specifications and intended use. The Redline ns and Amsoil FGL formulas also have excellent low temperature numbers with a higher viscosity index compared with regular Redline 75w90 and Amsoil severe gear which are both similar to Motul Gear 300 75w90 LS. Ihave an older 2000 Lexus LS 400 with open rear differential. Why do Amsoil and Redline both not recommend their gear oils that are also intended for manual gear boxes that require a gl5 gear oil? Is there some type of trade-off when a gear oil has a dual protection purpose? Have thought in past that gear oils with limited slip additive are not necessarily the best option for open rear differential-a Lubrication Engineer's article specifically mentioned to avoid lsd additive in open differential Yet-both Amsoil and Redline both recommend their higher viscosity gear oils with the limited slip modifier in the oil for vehicles with open rear differential. I am sure this topic has been addressed over the years-but wanted to get people's opinions. It seems that synthetic gear oils with the lsd additives have worse low temperature numbers-but are often recommended over synthetic gear oils from the Shell Spirax tree for most any car with open rear differential.
 

gathermewool

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Joined
Jan 9, 2009
Messages
9,283
Location
New England
For an open diff, the lube likely doesn't matter much, so long as it meets the OE spec, e.g., GL-5 75W-90. LS additive has zero effect on an open diff. Lack of friction modifier, such as with Redline's NS offerings, may be good for a tranny requiring a GL-5 lube, but has zero effect on an open diff. An open diff is the least needy of all gear sets.
 
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