SOT: high mileage motor oil, dino vs blend vs synthetic Options

[fixer34]

I know there have been multiple discussions about the 'best' kind of oil to use for our 4's & 6's, but I could use a little education on oils in general.

My daughter has a Chevy Trailblazer with 140k+ on the clock. She usually has one of her 'guy friends' change her oil because she doesn't trust the oil change shops and doesn't know what kind of oil they use.
Straight dino oil is getting hard to find and/or as expensive as synthetic. But I seem to recall from years ago that it was not a good idea to go from dino to synthetic without flushing out the entire engine. Still true?
And what about the 'blends'?

My DD uses synthetic and I just take it to the dealer/local shop for a change.

Words of wisdom are welcomed.

[Mikey914]

I had heard the same story on synthetic.
The way it was described to me was, the new synthetic oils work on a smaller level. Think molecular. They are designed to penetrate into the metal and allow the lubrication at smaller scale. Think softball size to BB sized. If the seals have any cracks that are currently holding under the larger size, the new smaller material can seep out. I would't change if my engine was tired, but you may be able to get away with it if in't in pretty good shape.

So the story goes.