I have nearly 500 tach hours on my IO-550 EXP from Performance Engines.  I have an Andair air/oil separator.  I keep the oil level between 5 and 6 quarts. I can pull the throttle back to near idle, if needed when descending, without getting oil on the belly.

 

The only time I ever had oil on the belly was during initial flight testing when I was doing zero G maneuvers to calibrate my angle of attack indicator.  Other than that time, all I get on the belly is an accumulation of dirty exhaust that is slightly oily that I clean off every six months or so.

 

I don't think your experience is typical, although not unheard of. 

 

Good luck tracking it down,

 

Dennis

Legacy, nearing 500 hours