Berryman B-12 Chemtool Fuel System Cleaner (part #0116) is completely safe for use in all gasoline engines at the maximum recommended concentration of 1 oz/gal. The only way the fuel filter could become clogged is if there were substantial fuel deposits in the gas tank that gummed up the filter; however, we’ve never had a report of such an occurrence. As for using the product in the crankcase, B-12 Chemtool is extremely effective in the removal of gum, sludge, and other crankcase deposits, but it should only be applied immediately before an oil and filter change. However, there is a caveat. If sludge or carbon buildup on worn oil rings has been helping to seal the crankcase, then applying B-12 Chemtool could actually cause the engine to begin smoking even if it didn’t before. Removal of varnish and carbon buildup can also affect tolerances in old, dirty engines and cause engine noise. These are very uncommon occurrences, but please be aware of the possibilities. If you would like to use B-12 Chemtool as an engine flush, pour into your cold crankcase approximately 1½-2 oz of B-12 Chemtool per quart of oil capacity. For instance, if your vehicle holds 5 quarts of oil, then you should use 7½-10 oz. of B-12 Chemtool. After you’ve added it to your cold engine oil, start the car and run it at idle 10-15 minutes or until the oil is warm. Then change the spent oil and replace the oil filter.