Well for Honda engines it's not really required because of the extremely strict tolerances, but I switched at a relatively high mileage (150k). Some say this is bad because the change has a tendency to deteriorate seals and cause oil leaks, but this is what I've found: My car's recent (last 10k miles) tendency to burn about .5 quart/1000 miles has completely stopped. I just took a 600 mile road trip doing very high speeds after driving about 300-400 miles around town and on short trips, and I did not burn a single drop of synthetic. It's gotten dirty a bit early because I think it's loosened some debris that regular oil didn't. I use Castrol full synthetic.