James, Steve is right, to design for the worst case scenario, such as a heavy oil truck, repeatedly parked in the same place. Also, when you order your mix, order the driest you can handle, the drier, the stronger, once cured. The drier the mix, the harder to spread, and the more strong arms you will need to man shovels to assist. Also, while the 8" thickness may be a bit much, and you probably can get away with six, remember that correct placement of the rebar and wire fabric is essential; for maximum strength. Good luck on your project.
Just as an aside, considering the cost of concrete, have you considered coble stone on a properly prepared (mechanically tamped) base of compacted roadbase, gravel and sand? Just a thought.