Dozens of threads on this very topic. Use the search function at top of page and check them out. About as many opinions as people commenting. Everyone has a story.

Here's mine: I have changed about 8 cars and two or three motorcycles over to synthetic (Mobil-1) at mileages ranging from near-new to 250k+. No leaks that weren't already there and those did not get worse. No special technique, just convert and go.

Go to Mobil's website and read up on it - you will find that they state that no flushes or other precautions need be taken when changing over to Mobil-1. And GM issued a service bulletin a few years ago strongly advising against engine flushes.

Conventional oils today are so good there is not a lot to be gained by using synthetic unless you are comfortable with extended oil change intervals, or - on low usage vehicles - leaving it in the engine for a long time. This describes me.

A recent Volvo bulletin to service managers mentions 10,000mi oil changes when using synthetic. That's my interval on my high-use vehicle.
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Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.