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API GL-4 vs. GL-5 compatibility

The SX4 MY09 owner manual strongly recommends the GL-4 75W90 gear oil. My dealer put the GL-5 (75W90) as the GL-4 was declared “obsolete” for North America!? Is it safe for the synchros/bushings? I do not believe so but need any expert’s opinion. There is a field in some technical data sheets labeled “Copper Corrosion - 1B”: is this the only specific characteristic/data to look for/compare?


From what I can tell GL-5 has replaced GL-4 in the US, and shouldn’t be a problem. More info here, several good links:

My VW also requires GL-4 because GL-5 has sulfur that will attack the brass trans synchronizers. If the GL-5 that was used in your trans specifically states that it is compatible with brass synchros then there may have been some updating since my VW was made. You might want to Google the various specifications claimed for the lube used to see if one covers brass compatibility.

For many years GL-5 was for differentials only. GL-4 was correct for manual transmissions. Topping off with a few ounces of GL-5 was acceptable but filling from dry was not. But things change, I guess.

Wha Who is correct…My 98 Nissan had a very strong warning about using GL-5 in the transfer-case. The transfer-case had brass…and the GL-5 would eat the brass parts and destroy the transfer-case.

GL-4 and GL-5 are NOT compatible.

According to ‘bobistheoilguy’:

“GL-5; Specified for hypoid gear service but with shock loads and severe service operation. Usually meets Mil-L-2105D and in most cases, is the multipurpose automotive gear oil. Most 75W90 to 75W140 grades meet the GL-5 classification. This grade has a high level of Extreme-Pressure additives that could be mildly corrosive to nonferrous parts, such as brass, bronze and aluminum parts. Most of the modern GL-5 lubes contain metal deactivators that prevents attacks by the extreme-pressure additives. In addition to EP additives, these lubes contain rust inhibitors, defoamants, friction modifiers, thickeners, and Viscosity Index Improvers.”

So it sounds like ‘most’ GL-5 oils might be OK. But that’s just from this source.