Go with the owners manual recommendations. DO NOT switch to pure syn oil on an old engine unless you are prepared to change your oil every 1500 miles for the first year. Syn oil will super clean an engine that was using dino oil. It in itself doesn't cause seals to leak. It simply cleans away all the sludge that was helping to seal the worn seals. Syn oils bennefits are: No sludge. No varnish. And, your engine will never get hot enough to burn syn oil. Thus no sludge, no varnish. I don't know what the ratio is in the "blend" you're using but, it's probably about 10 percent. And that should be okay. Syn oil was recommended in 'vettes from '92 on. Not because they are special engines, but because they could delete the oil cooler and save a little weight. Again, syn oil won't burn or vaporize. (Unless you're about ready to chuck a rod). I've torn many 150k engines down and a blind man can tell if it was usning dino oil. You could make a Christmas candle out of the waxy sludge that comes out of them. A syn oil engine with that milage looks like it was put together the day before.
Now, unless you're planning on buying a brand new car and keeping it for twenty plus years, and it's not a late model 'vette, go with dino.
MTCW Good luck.