Originally Posted by SrDudley
You do not want to put automatic transmission fluid into your manual gearbox that's what gear oil is for. Completely different....
First of all, the slang term "gear oil" is often confused with conventional 75W-90 or similar gear lubricant which is GL-5 certified for use in certain older manual transmissions and hypoid gear differntials. That heavy duty stuff will work fine in a 60's muscle car or the differential of a new 5 liter Mustang, but it will literally eat and destroy yellow brass synchro rings and other parts found in any modern manual transmission built since about 1975. It has the wrong additives for a modern manual transmission. Gas mileage will suffer, the transmssion will feel baulky upon shifting and it will likely fail.
Are you aware that automatic transmissions have gears inside?
Ford SVT service documentation instructs SVT Focus owners to put Mercon V into their manual transmission based upon Getrag Engineering recommendation. Many manual transmissions designed in the 80's until about 5 yrs ago were specifically optimized for running ATF. Getrag's whole manual transmission product line used ATF until the late '90's when they started the switch to mfr-specific manual transmission lube for the M3 Getrag 6spd and some other's. You shouldn't just arbitrarily substitute a gear lube assuming it will be better than what the OEM specified. This is what foolish Toyota Supra owners did in the 90's. Those Getrag 6spds that were built for Dextron quickly failed on redline manual transmission fluid. Owner's manuals describe what lubricants to use. Mercon V is not just for automatic transmissions. It's used for power steering systems too.