There is a continueing debate that some full synthetics may tend to slightly shrink old seals. Those that have a high level of Group IV base stock are the only ones that might be effected. This includes Mobil 1.
I switched to full synthetic at about 190,000 miles. I had a very slight leak prior to the switch. It seems to be about the same.
When I switched, I also moved from 3,000 mile oil change intervals to 5,000 mile oil change intervals. On the 3,000 mile oil change interval, the oil is still full when changed, and even now it still show full at 3,000 miles. Using a 5,000 mile interval, I need to add about 1/2 quart at 3,500 to 4,000 miles.
It is my belief that you can switch between synthetic and dino any time. That is also what the oil companies publish. If you are concerned, you may want to try either a high milage synthetic (Valvoline Max-Life full synthetic) or try a synthetic blend instead of a full synthetic.
