Based on the poll results from the poll that our hosts are running, most people here use either Castrol or Mobil in their cars. I would suspect those using Castrol are using convential oil and those using Mobil are using Mobil 1, which is a full synthetic. If you plan on changing your oil every 3K, then Castrol GTX will be fine. Since my engine is a diesel, I use Shell Rotella every 3K.
To answer Matty190e's question. It really depends on well the car is maintained and how it's driven. Even for a tranmission I think it would be best to go easy on it until the tranny fluid has a chance to fully warm up. I routinely see people put their cars into reverse and before coming to a complete stop, throw it into drive. This would be quick way to kill a tranny. So I guess what I am saying is be kind to your machine and she'll return you in kind. You can go to the MBCA web site and find an article about the average life of different components on the car. Their web site is http://www.MBCA.org.