Amsoil Engine and Transmission Flush 16oz Pre-Service Cleaner

Amsoil Engine and Transmission Flush (part number FLSH) is a 16-ounce detergent concentrate you add to the used oil or automatic transmission fluid right before you drain it. You run the engine or cycle the transmission for a short time so the cleaner loosens and suspends the varnish, sludge and gum that build up on internal surfaces, then it all drains out with the old fluid. One bottle treats up to 6 quarts of oil or fluid.

On an engine, pour it into the crankcase through the oil fill, idle for 10 to 15 minutes at operating temperature (do not drive on it or load the engine), then drain, change the filter and refill with fresh oil. On an automatic transmission the same idea applies, but only do it if you can also drop the pan and replace the filter afterward: add the flush, let the transmission cycle through the gears at idle for the recommended time, then drain, install a new filter and gasket, and refill with the correct fluid. Servicing a transmission that has never had the flush cleaned out will pull loose more debris, which is exactly why the filter and pan come off in the same job.

The point of a flush is to get a fresh fill working on clean surfaces instead of leaving old deposits behind that immediately start contaminating the new oil. It is aimed at high-mileage or neglected units where the fluid comes out dark and thick, and it is a common step before switching to a full-synthetic oil or fluid so the new lubricant is not fighting yesterday's sludge.