Did you use assembly and cam lube?
If so maybe that is what your seeing, I see ARP rod bolt lube in the bottom of the oil pan of every motor I've assembled and look in the oil pan later
Probably takes 5 or 6 oil changes to get it out of there if you don't remove it your self after breaking in the motor
I have heard of some synthetic oils doing weird things., especially in race motors
I owned and flew a air cool airplane for over 700 hours, I switch the oil from regular to Mobil AV1 synthetic aircraft oil, it ended up slugging that motor up in less than 300 hours, we where required to change the oil and filter every 50 hours and the complete airplane had to have a mandatory inspection once a year if you flew it less hours per year than the manufacturer mandated for component inspections. The motor was slug free on the first inspection as it had sat for several years before me buying it so I had it inspected and remove a rocker arm cover from both sides of the motor to look inside for corrosion from it sitting that long, it was hangered at the north end of the San Franscico bay. No corrosion or slug
Most of the trips we flew in that airplane where over 1 hour, usually 2 Hrs.+ per leg, my wife kidney where usually the limiting factor on how long in the air before landing