Hi All car Guru Esp Gazzuz


I was advising a friend who has been using 20w50 total oil of #4000 to stop using it and be using spec written on oil cap (5w30), he got 207k mileage now from almost 155k mileage, if he continues using it till 250k to 300k mileage, what might likely happen to the car engine, he intend to know the reason he should use what manufacturer prescribe for the car and the danger if hes not

Thank you