Him saying to get the oil lab tested wouldnt put my mind at ease - who the feck is gonna go to that length, or know where to get it tested - i sure as hell wouldnt know without having to google lots and no doubt part with even more money and waste even more time - he knows that too so IMO its just a fob off.
For what a few litres of oil would have cost them i'd have wanted them to have changed it again as a gesture of good will - in real terms its gonna cost them next to nothing and you might then actually go back, instead they've lost your custom for good and as a result of you posting the outcome here i dare say a number of other potential customers will now not touch them with a barge pole - the overall cost to them is considerable really and short sighted on their part.