I recently purchased the 6 litres of oil for my forthcoming service from Opie oils. I'm very pleased at paying £60 instead of the £120 the dealer wanted to charge me. However on looking at the bottles in detail I notice that each has a date stamp ranging from April 08 to March 10. Now I presume this is the date of mnanufacture and not some sort of expiry date like on a bottle of milk! Should I be concerned that some of the oil is over two years old or does it "keep" if in a sealed container?