Just ask for a bottle of synta silver oil, they should know what you're on about. If they are complete muppets then you'll have to tell them it is made by quantum, its 10w40 and comes in a big silver bottle

Make sure you get a proper oil filter and the little copper sump washer too

Bear in mind that if the rattling doesn't stop either the tappets have had it or its your cam chain thats rattling.