Oil in our air-cooled engines serves two purposes - lubrication and cooling therefore it takes a heavier than normal beating. Individual car owners decide which oil to use with the best intention of keeping their engine protected. I use a regular grade 40 oil but keep the oil change interval at 2500 km. The care I recommend is that oil be purchased from a reputable outlet. Synthetic oils are great for modern cars and actually offer benefits due to the long drain intervals, superior lubricating and cleaning properties. However, in my personal opinion synthetic oils are a waste of money when it comes to regular air cooled engines especially if you are going to change them at a shorter interval. running it for a longer interval may actually be counterproductive for the health of your engine.