I also have noticed it runs smoother ,both driving and at idle. 2 Blazers with syn. 1 at 250,000 and 1 at 160,000. No leaks. When going from reg to syn. I would change it and then at about 500 miles change it again, there for cleaning the internals. plus after this you can easily go 6000 miles between changes at normal use

 

I would change it around 6000-7000. Even after that time it would come out a golden brown.
I think they should do like they did with transmission fluid and make all oil synthetic. JMO

 

I run Motul eco energy in my truck..it's 100% synthetic oil..ester based...haven't had any issues...the 4.3 really likes it...so far no leaks even with new oil lines..oil pressure is steady...40 to 60 driving and 20 to 30 idle

 

I decided to go with 10W30 on my latest oil change. Engine a little quieter, oil pressure up 5-10 psi at both idle and high rpm and engine temp 5-10 degrees cooler.