First service at 100 miles is important due to everything being broken in at that point. Rings and gears can produce some shavings and such during the seating process and you don't want that staying in there. Every 600 is like the every 3000 for a car, some engineer must have determined that this was when the oil viscosity started breaking down or something. I went with synthetic on my second service and am sure that I'll be draining out perfectly fine oil but better safe than sorry. The filter is more important than the oil at these intervals really.