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Old 07-18-2020, 01:30 AM   #18
JohnH
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Join Date: Jun 2020
Drives: 2020 BRZ ST RS
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I don't know specifically about our cars, but I am a liscenced tech and if a car doesn't specify the use of GL5 and you put GL5 in it, you will ruin the soft metals/yellow metals. Those are what your synchro rings are made of. The additives in GL5 attack yellow metals over time, causing premature wear. Worn synchros give you the symptoms you are speaking of.

I also know that there are some modern GL5 rated oils that are made NOT to damage yellow metals, so you could use those.
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