Best option is Amsoil full synthetics, i liked it so much i started to sell the product as well.
The better oil you use the less friction the engine contains. Yes you will notice an increase in power and mpg but the percentage varies. I get a mile per gallon better on my vehicles and so do my customers.
When making a switch the best thing to do is an engine flush. Amsoil has an engine flush that runs for 15 minutes before you change it. Run it while its hot then drain the oil and add the new stuff. You want all the old petro out because it will shorten the life of the synthetic.
As long as you have no leaks and its a fairly sound engine you will have no problem switching to synthetic. The only reason people ever do is because synthetic is a real thin oil with cleaners in it and it may clean out a seal that was plugged with sludge.
Hope that helps, take care.