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Originally Posted by 123Britt
Interesting that you can find users saying to only use GL5 and others GL4.
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The OEM oil in my car was / is GL4.
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Redline says GL-5 will work and protects better. My thoughts are that GL-5 is a better diff oil but could end up being corrosive if subjected to higher temps and that is why BMW went GL-4.
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OEM fluid is stupid expensive.
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GL4 vs GL5 is normally only a problem in manual transmissions with brass/bronze synchros. The GL5 high sulphur content can corrode those metals. In a diff, either GL4 or GL5 that meets the weight and additives requirements should be fine.