99 ML430 what is correct engine oil?
 

I haven't been on here for quite some time as I was between Mercedes for a couple years when money was tight. Now I find myself with 3 in the stable again. Last fall I picked up a super clean 99 ML430 with 88,000 miles from a Mercedes dealer, they sold the truck new, it had always been serviced at the dealer and the original owner traded it in on a new one. I got all the service records and it seemed no expense had been sparred in its up keep. I was kind of disappointed that the Service Center's printouts were not very detailed, just a work order # and very brief description and a total amount. Amazingly the CARFAX was more informative about what had been performed. Anyhow the truck came with a fresh oil change when I bought it, the dealer had been using Mobil1 5w-30 at around 10000 mile intervals.
The truck now has 95000 miles and I would like to change the oil at intervals that are easy to remember, IE 95000,100000,105000 you get the idea, I do this with all my other vehicles as well.
I was at the parts store the other day an they had Mobil1 on sale (almost $3/qt off) so I grabbed 9 qts of 5w-30 since that was what the dealer had been using. I got a Genuine Mercedes oil filter from the dealer.
Wanting to see the oil filter remove and replace procedure I began searching online and found out most folks were using 0w-40 in their vehicles.
Should I return the 5w-30 in exchange for 0w-40 or is it that important?? And what of the 10000 mile interval, my Father in law (a retired GM mechanic) always swore by 3000 mile intervals, only a few years ago did I convince him to switch to the 5000 interval. 10000 makes my cheeks tighten, somewhat. Although, since the oil is so expensive and the truck holds almost double the amount of most other makes with similar sized engine, it must be OK.
Advice appreciated, Thanks Eric:)

99 ML430-95,000 miles
87 300SDL-250,000 miles
87 300D-T-65,000 miles
98 Dodge Ram/Cummins-370,000 miles
93 Mustang LX 5.0 convertible-90,000 miles (just for fun)

Eric, just get an oil that meets MBz 229.5. That spec encompasses everything for your car.

I suppose Mobil 1 0W40 is easiest to locate. W*mart now has 5qt jugs for $25.

Mobil 1, 0w-40 is the only Mobil 1 oil with the MB spec listed in the back. I change mine at every 10k miles.

I never really bought in to the 3000 mile oil changes.
What I have used for a long time is a pretty simple formula, and the basic formula I have considered applicable for "regular" non-synthetic oil, and that is 1000 miles for every quart of capacit of the engine, so a 5 qt car is 5000 for example. Bear in mind the Mercedes V8 holds 8.5 quarts. You put that muck oil down the bunnyhole you should expect something in return and it sure ain't a 3000 mile oil change!
For as much as you pay for synthetic I would expect also something in return, I would feel confident in running that synthetic even longer, in general twice as long, but with even a tight wad like me would feel pretty nervous about 17,000 mile oil changes, unless you were racking up constant highway miles, like 24/7 around the clock driving. But sure wouldn't feel that 10000 would be that much of a stretch for that oil.