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.

[theer]

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?

That is one of those myths that just won't die. You can add any motor oil to any existing motor oil. They even sell dino/synth blends.

The leaks in old motors concept sounds a bit more believable, since, as I was told, the synth oil molecules are smaller & slipperier than dino. This, too may be a myth.