I accidently put a quart of 10/40 Quaker State motor oil in my 1991 Ford Mustang 5.0 with automatic transmission. I read in my owner‘s manual that the transmission capacity is 7.9 quarts. I know it‘s not good, but unless I take it to a shop where they can flush the whole thing, torque converter and all, I can‘t undo the stupidity of grabbing the wrong bottle. Are there any transmission experts here who can give me advice. Will one quart out of 8 be catastrophic?
Sorry but the 1989 Tbird engine is not compatible with the 91 mustang
Just take the trans pan off and do a trans service (filter and fluid). As long as you don't drive it it'll be OK. After about 1000 miles do a trans service again and you should be OK.
Quick answer, Yes! The seals in the transmission are not designed for the chemicals in motor oil or vice versa. Your clutches and plates will start slipping because motor oil is designed to provide a lubricating film. They will burn out too. It's simple economics, do you want to pay now for removing a mistake or do you want to pay later for a whole transmission repair? Hope you aren't driving around in this mistake. You can drain the transmission yourself by using one of the cheap syphon pumps from an auto parts store. Make sure the syphon tube will fit down the dipstick tube. Remove the dust cover under the torque converter and turn it over until the drain plug is on the bottom back edge. Drain it. Do NOT have someone tap the starter to turn the torque converter while you are under the car. Safety first. I strongly suggest you take it to one of those oil change places and let the people who do this for a living handle it. It's cheap considering the other options.