I've got a new 2009 Express 3500 with the 6.0 gasoline engine. Owner's manual recommendations aside, at what mileage would you do the first oil change? At what mileage would you make the change to synthetic oil? I presume the oil percentage life remaining can't differentiate mineral and synthetic oil so how does one accommodate the longer intervals between changes? When changing to synthetic do you stick with 5W-30 weight? Anything else about this I should know but don't know enough yet to ask?