A friend of mine asks if M1 FS Xw40 or 5w50 would work in his specific CNG engine use case. I don't actually know the whereabouts of CNG workings and its demands on engine oil. Should he go FS or stick to conventional 20w50 as before. Engine is 1.6 port injected and naturally aspirated, he's very cost aware (that's why he opts for CNG and conventional lol). Our debate goes that a synthetic will squeeze a longer OCI and the cost might be just be slightly higher but retains more benefits and advantages long term that a conventional might fall short of offering no matter what. Please help me win this debate lol.