Stick to 5w40, your engine doesn't require a thinner oil, plus 75k is quite a bit of mileage. Stick to this oil, as for the flush, it helps clean the system, the procedure is to first, empty the chamber, then put in 2 litres of oil, start the engine and keep it like that for 5 mins, rev it once or twice, then turn it off and remove the oil. Flush is only required if the chamber is dirty and it changes the oil colour to a dark one. If your oil doesn't change colour, flush isn't necessary