Hi Chicago - what are you like with rabbits
My view for what it's worth
As Musky implies, only way for oil to get from the primary to the oil tank is via the sump, has to be suction involved and it's drawn past an oil seal or through a bolt hole.
Also an inch and a half extra in the oil tank is almost more oil than should be in the primary I'm thinking.
Other explinations - petrol - doubtfull and dangerous, - water - just doubtfull - ectoplasm (or someones taking the P---)
No it's a good one this.
I'd suggest draining the primary case, go for a run (short one) then check the oil tank, will prove your right or not
( re reading my words it may seem it's me taking the proverbial, but not intended)