Your handbook should tell you if you have one. Generally, a 10/40 is OK. Doesn't have to be any of the fancy stuff, unless it's a diesel, then the oil has more detergent in it to get rid of the extra muck a diesel creates inside the engine and the oil has an extra high shear capacity, cos diesels are a little rougher on oil than petrol engines. I wouldn't go for the absolute cheapest stuff, as it won't clean the engine insides so well. Any mid-priced 10/40 will suffice.
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