Regular oil is not thinner than synthetic either. There is a LOT of mis info out there (and in this thread) about all of this.

Synthetic oil will be just as thick at a given temp as dino oil of the same spec.

 

here a site about motor oil http://www.atis.net/oil_faq.html
This site is pretty old, some spec of brand motor oil haven't been updated, But this should give you an understanding.
Changing from regular oil to full synthetic oil, but using the same grade you only changing the characteristic of the motor oil like flash point and etc...
All company oil are slightly different in spec, just depend what spec you don't mind having in your car.

I am using mobil1 full synthetic since my third oil change, as for my tranny(manual) after three years(23k mile) of using the same oil when i brought the car from the dealer, I just changed it to Redline MT-90 which is synthetic.

 

Go synthetic..im running mobil one since 3,000 miles..im now at 27k and shes doing fine, i just switched to the redline for the tranny at 23k and i love how it feels after that swap. Also mixing of conventional and synthetic wouldnt harm anything if they did, i mean they sell synthetic blends that are the two mixed..so dont worry.