60 days return. Buyer pays for return postage. If you use an eBay delivery label, it will be deducted from your refund amount.
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While topping up the oil at that frequency may be annoying and possibly expensive, there is generally nothing to worry about. If a car requires a 0w-x grade of oil, then it must be a genuine synthetic oil as it is impossible to make an 0w viscosity oil from mineral oils.