If you have no qualms about doing it then stop round the corner from the dealer, clean it all off the best you can then do the deal. Just make sure you wear gloves as if you shake his hand with an oily hand he might get suspicious.

No salesman is going to look under the bonnet let alone open the oil filler cap. So long as it starts and runs OK that's all he will be interested in.

For what he will offer you px on it it's not worth his while getting his hands dirty.

2p