I have a rebuilt engine with about 1,500 miles on it and I want to start running synthetic oil in it.  It is a everyday driven 71 olds cutlass with a 70 chevy 350.  I don't know what brand to use.  I usually use Castrol GTX which I like, but I don't know how their synthetic is.  Any help would be great.  And I'm taking this car on anti-tour this year.  

You might want to look at this web site it's interesting reading.  I went with Mobil 1 but I don't really have any concrete reason why.  Might be a good article for the mag. (hint hint)

http://neptune.spacebears.com/cars/stories/oil-life.html

Happy reading.

ANY syn. oil labeled as "full synthetic" or "100% synthetic" is light years ahead of any petroleum based oil in all properties!  Unless it states these two designations on the label, then it is a blend with natural petroleum based oils, which still is an advantage over traditional natural petroleum based oils. Before you use full syn. oils, you should correct any leaks that are practical to do so on a cost and/or involvement basis, and do a newly assembled motor break- in period with natural petroleum oil, THEN drain and switch to your synthetic. For any part of the continental U.S., 0 or 5 or 10W-30 is great or 10W-40 if heat REALLY concerns you. Amsoil and Mobil 1 are top choices, with Amsoil having the TOP filters in spin-on, cartridge and by-pass systems, and they back it with written proof!

I decided to go ahead with the Mobil 1. Thanks with all the help.  

Mobil 1 is great oil.  Corvettes, AMGs, and NASCAR can't be wrong!