Yeah, not sure where the "Twice as many oil changes" part comes in. I've owned a couple diesels in my time (including a CDI Merc).
They need their oil changed less often than their petrol powered counterparts.
As for MPG.. even if you just base it off the MB GL350 BluTech (different engine and tranny), that beast is getting 19/26 and it's a fair bit larger then the GC.
Their engine is producing the same HP and Torque, as such, with a lighter body and an extra gear, I see that 30MPG on the highway being within reach.
I cranked out the mathes a bit.
Keep in mind, as we don't know the EPA numbers for the EU CRD version, I had to do a % comparison from the SRT and apply that to the CRD... As such, these numbers should really be viewed as a "Worst Case" result.
V6 MPG (City): 16.0 MPG (Hwy): 23.0 MPG (Combined): 19.0 Fuel Capacity: 26.4 gallons Range (City): 422.4 miles Range(Hwy): 607.2 miles Range (Combined): 501.6 miles Fuel Cost: $3.17 Estimated Fuel: $3,336.84
V8 MPG (City): 13.0 MPG (Hwy): 20.0 MPG (Combined): 15.0 Fuel Capacity: 26.4 gallons Range (City): 343.2 miles Range(Hwy): 528 miles Range (Combined): 396 miles Fuel Cost: $3.17 Estimated Fuel: $4,226.67
SRT MPG (City): 12.0 MPG (Hwy): 18.0 MPG (Combined): 14.0 Fuel Capacity: 26.4 gallons Range (City): 316.8 miles Range(Hwy): 475.2 miles Range (Combined): 369.6 miles Fuel Cost: $3.43 Estimated Fuel: $4,900.00
CRD (Current 5 Speed using EU numbers) MPG (City): 22.8 MPG (Hwy): 32.7 MPG (Combined): 28.0 Fuel Capacity: 26.4 gallons Range (City): 601.92 miles Range(Hwy): 863.28 miles Range (Combined): 739.2 miles Fuel Cost: $3.89 Estimated Fuel: $2,778.57
CRD (Current 5 Speed using Estimated EPA numbers) MPG (City): 24.3 MPG (Hwy): 25.3 MPG (Combined): 24.7 Fuel Capacity: 26.4 gallons Range (City): 642.83 miles Range(Hwy): 666.6 miles Range (Combined): 650.79 miles Fuel Cost: $3.89 Estimated Fuel: $3,156.04
Here's my estimates on the 8 speed, with a 10% boost to both City and Highway (I find the City numbers dubious, but.. whatever).
V6 (Estimate with 8 Speed) MPG (City): 17.6 MPG (Hwy): 25.3 MPG (Combined): 20.2 Fuel Capacity: 26.4 gallons Range (City): 464.64 miles Range(Hwy): 667.92 miles Range (Combined): 532.74 miles Fuel Cost: $3.17 Estimated Fuel: $3,141.80
V8 (Estimate with 8 Speed) MPG (City): 14.3 MPG (Hwy): 22.0 MPG (Combined): 16.9 Fuel Capacity: 26.4 gallons Range (City): 377.52 miles Range(Hwy): 580.8 miles Range (Combined): 445.62 miles Fuel Cost: $3.17 Estimated Fuel: $3,756.04
SRT (Estimate with 8 Speed) MPG (City): 13.2 MPG (Hwy): 19.8 MPG (Combined): 15.4 Fuel Capacity: 26.4 gallons Range (City): 348.48 miles Range(Hwy): 522.72 miles Range (Combined): 406.85 miles Fuel Cost: $3.43 Estimated Fuel: $4,451.37
CRD (Estimated with 8 Speed Estimated EPA numbers) MPG (City): 26.8 MPG (Hwy): 27.8 MPG (Combined): 27.1 Fuel Capacity: 26.4 gallons Range (City): 707.11 miles Range(Hwy): 733.26 miles Range (Combined): 715.87 miles Fuel Cost: $3.89 Estimated Fuel: $2,869.12
Keep in mind, this is an estimate cost on the combined cycle with 20,000 miles per year. V6, V8 and Hemi - Unleaded Petrol SRT - Super Unleaded Petrol CRD - Diesel
Why the weirdness in the difference between the EU and US MPG numbers? Simple really, they test their highway and city differently from each other, producing a sizeable difference (6MPG difference on the SRT Highway cycle alone).
At anyrate, this is my best conservative guess. I wouldn't be shocked to see higher MPG numbers than these posted when the time comes.