I want to change the old oil in my TT. A friend suggested adding ATF to the old oil and running the engine a bit first. Since the ATF has a lot of detergent it would help clean out the gunk. Does this work? Could this be harmful to this old engine? Is there a better method of cleaning out the old deposits and tar like oil? I don't have clean environment, like a garage, where I can open up the engine. Thanks.
Not going to do damage but it could break sludge loose and plug your oil pick-up tube. Kinda depends on how gunky the engine is and how frequently the oil has been changed.
I think it would make more sense just to run the detergent oil of your choose and keep changing it on a regular basis.
Adding ATF to the oil and running it for a short period of time prior to the oil change just to "rinse it out" isn't going to do much, in my opinion.
I think it's good to open up all three inspection lids on an unknown engine to have a good look - and if there is a large amount of sludge, cleanup is needed. The oil pipe can be blown through with compressed air (and a piece of tube) with the bottom plate under the pan off.