Isopropyl Alcohol

I've used it on my old f-100 before. worked fine. I used the 90% stuff that you get at the drug store tho. If he tryed it once & the problem still lingers, I would drain & flush the tank.

When I worked for the army railroad we did this quite often. Our old locomotives were 2 stroke, V16 diesels & they liked to blow oil out the stacks. About once a month, we dumped in 5 gal. of alcohol to every 2000 gal. of diesel. Burns super hot when mixed with diesel. Just hot enough to burn the oil out of the stack, but not hot enough to hurt a 9000 c.i. diesel engine.

EMD 16-567C prime mover. (16 cylinders @ 567 C.I. each), 2 cycle, roots blower scavenged.
1900 gross hp,
good luck!