This is from the mobil 1 site might offer some insight. Switching From Conventional Motor Oil to Mobil 1
There seems to be confusion about how to make the switch from conventional motor to synthetic. I am a do-it-yourselfer. Could you provide instructions?
-- Joe D., Miami, FL
The instructions are very easy to follow: Simply change the oil as you would normally. Any “confusion” is just myth. You can switch from conventional motor oil to Mobil 1 synthetic (and back again, if you want), without following any special procedures.
The only exception to this is with a higher-mileage engine that has never used synthetic motor oil, or with an engine that has used conventional motor oil and been poorly maintained. In these cases, you should still follow the same basic oil-change procedures (drain the old oil, remove the old oil filter, put in new Mobil 1 and put on a new oil filter), but you should follow a regimen of one or two shortened oil-change intervals. For instance, let’s say that your regular oil change interval is 5,000 miles. If you’re switching to Mobil 1 under the circumstances mentioned above, make your next Mobil 1 oil change in 2,500 miles, your third Mobil 1 oil change 3,500 miles after that, and then follow your normal 5,000 mile oil-change interval. The reasoning behind this staggered interval is that a high-mileage engine, or one that has seen infrequent oil changes, will likely have a considerable build-up of sludge and deposits. Mobil 1 will help clean the engine as you drive, but it will have to work much harder in a very “dirty” engine, and so it is best to change the oil more frequently for those first few thousand miles. After that, you can rest assured that Mobil 1 is continuing to keep your engine running clean and well lubricated for mile after mile.