W261w261

I'm looking at a 106k B5 1.8 that seems to run fine, no oil pressure light or running issue that I can see. However, the owner says he's used semi-synthetic oil while he's had the car. Is this a deal-killer, as why take the chance that there's sludge but it hasn't gotten to the critical point yet, or could I do a milder sludge-cleaning treatment like Seafoam in the crankcase? Or best just to keep looking and find someone that's used full synthetic? Thx.