That's what caused the same problem on my dog's Sportster
The engineering answer is unlike a lot of other brands, the dipstick does not screw into the engine case. Harley dipsticks often have chrome-plated skull heads which do not to fix the problem, but they sure are shiny and macho.... I guess the thumbs downs are for stating the facts. Oh well.
Combustion blowby from worn out piston rings, or burned pistons.
Your crankcase pressure is too high. Check/clean PCV system or case breather (whichever it has).
crank case vent ....( P C V )
