What is Service G in E240
You can try to put a dipstick of some kind in it and simply look at the oil. If it is obviously full of carbon, change it. But that is far from scientific. Sending it off for analysis is possible, and some swear by it, but I wouldn't bother.
Think of the question from the perspective of what will do the most harm and the remaining value of the oil in the engine now:
a) Changing the oil before it is necessary won't hurt anything much except, initially, your wallet. If you have 4,790 miles to go, that's about half the life of a synthetic oil. That suggests that if you change it now you will be throwing away at most 50% of the residual value of the oil.
b) Leaving it another 4,790 miles. If the oil has less than about 5,000 miles on it, that would be no problem, but you don't know.
My approach with a purchase of a vehicle with inadequate documentation of maintenance is to change all fluids and restart the clock. Engine oil, transmission fluid, diff oil, brake fluid, maybe coolant. This costs money but not as much as inadequate maintenance.
(BTW, do not change the oil every 2,000 miles as some people do. New synthetic oil contains detergents to clean the engine. These detergents are consumed in the first few thousand miles. It is a bad idea to change these oils too frequently, because the full lubricant capability is compromised a little in that initial period by the detergent. Mobil 1, etc, can go for 10K and should not be changed too frequently. Too-frequent oil changes give you a squeaky-clean, but perhaps faster-wearing, engine and fail to take advantage of the long-life oil technology our great-grandfathers would have been envious of.)



