The Logic behind using Diesel oils is that they are designed to cope with much, much higher amounts of carbon deposits in the oil itself because of very high compression in diesel engines. More compression = more carbon forced paced the rings. Scooter engines are not, for the most part, using high tolerances like modern car engines and run fairly high compression making the oil dirty (Carbon deposits) quickly. As CE stated, they also only have a quart of oil and for the most part no oil filter other than a screen. As with anything, Kymco would not recommend using something for no reason and they indeed recommend using Diesel oil in the scoots. 10w40 winter, 15w40 summer IMHO depending on where you live. Mild climate 10w40 year round, hot summers 15w40, very very cold climate 5w40 or even 5w30 if you can find it. Cheers