oil driping on spark plug boot

strange new problem. The number six spark plug boot has oil dripping on to it. I do not know where it is coming from. The rocker arm covers are not leaking so I am at a loss.

any ideas?

91 4.3L C1500. 170,000 miles.

may be coming from the dipstick, my racecar used to blow oil out of the dipstick all over the top of the motor.