Gasoline will leave some kind of chemical residue on the metal substrate causing oil not to adhere... GM had a huge problem with this very issue in the past when rebuilding motors at dealerships gas was used as a solvent which then caused problems with wash down after the rebuild scoring etc was seen.... I would NOT use gas as a solvent if you must kerosene or diesel, but as I said there are much better chemicals that will do no harm/etching of the metal. You should have plenty of time to do this so use the soaking method, let the chemical penetrate, and break down the oil...
I have found about the only thing that removes varnish is mechanical removal which you don't want to do as this will cause more problems ..such as using a wire brush will scratch the metal causing the next oil/heat cycles to be even worse. Cleaners with caustic soda, or sodium hydroxide does an outstanding job of clean sludge, caked on oil, and even some varnish... but please research this as to not harm you engine... sodium hydroxide, caustic soda will erode aluminum in a bad way. Zep one of the best cleaners I have ever used, also Castrol Super Cleaner.
99% alcohol is also a outstanding cleaner which has pretty much no effects on metal, but can on rubber/plastic parts... You are going to have to research this, and all the materials you have to clean. When you get done with this I think you will see my point about all the cleaning additives for oil pretty much are worthless, all they do is thin the oil out (kerosene effect)