2005 Buick rendezous 190,000 miles would it make difference if I start using synethic oil?

What are you trying to gain? If I were stick with the type of oil that's been recommended-

I agree with Ras. there's nothing to gain at this point and you could actually do some damage by switching now.

Wrench, some islanders especially the young ones they says it makes their car run faster-is that true-lol

They also think a different air filter will gain 35HP.

maybe purple oil but thats minimal