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Will Gear Lube Viscosity Difference Cause Problems?
 
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Question:
Will Gear Lube Viscosity Difference Cause Problems?
My vehicle is a 1997 Toyota Supra non-turbo and the replacement for differential oil says that I need Hypoid GL-5 SAE 90 gear oil for temperature above 18°F or 0°C. It says that GL-5 80W-90 is ok but the SAE 90 is preferred. When I drained the old oil, the old oil was much thicker. I went to Kragen Auto Parts and the sales people recommended me getting the Mobil 1 Synthetic LS 75W-90 rather than Valvoline GL-5 80W-90. I went ahead and swapped out my old oil with the Mobil 1 but the Mobil 1 is not as thick due to the weight difference. My question is whether the difference in the viscosity will have long-term damage to my limited slip differential? I know that using an oil too thick or too thin can both cause problems. I don't really race my car since I have 172K miles on it now and just want to prolong the life of the vehicle as much as possible. Thanks in advance for taking this question.
-- Lex Conby, Los Angeles, CA
Answer:
The sales person made a great recommendation.  Mobil 1 Synthetic Gear Lube LS 75W-90 is an excellent choice. It will still provide the right operating viscometrics since it is an SAE 90 grade at temperature.  Plus it will provide good fluidity even on the coldest day of the year. The reason the drain oil looked thicker was probably that it was severely oxidized. Our Mobil 1 gear lube will provide outstanding resistance to oxidation as well.  
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