What's the best grade engine oil to use in my car?

My petrol-engined Toyota Yaris Verso runs happily enough on 5W/30 semi-synthetic oil. It returns about 1600 miles per pint of oil. With 135,000 miles on the clock, would the engine benefit from an additive such as STP Oil Treatment? Or would semi-synthetic 10W/40 oil be more appropriate?
Given that the engine is running okay and you change the oil at the correct intervals, we would not see a reason to add anything else. As for the grade, 5W/30 is for -29 C to 10 C. The W in oil spec refers to winter and the oil viscosity/flow at a low temperatures. The lower the W number the better the oil cold temp/cold start performance. The 40 number is referring to the viscosity limits at 100 degress C. So a 30 oil is thinner than a 40 oil. Given the UK weather a 10W /30 oil would cover a temp range of -18C to 38C which we would consider about right.
Answered by Alan Ross on

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