Engine Oil
If you are running an older G, there are some good deals to be had on genuine MB, fully-synthetic engine oil. I think they must be having a clear-out sale:
For Petrol and Diesel:
You can get a 20 litre drum of fully-synthetic, 5W-40 for £55 delivered. This is the MB 229.5 spec oil:
It seems to be the 'Formula-M' (Mercedes) version of Mobil-1. Not quite as high a Viscocity Index as their 0W-40, but close:
https://www.mobil.com/english-us/passenger-vehicle-lube/pds/glxxmobil-1-formula-m-5w40
For Diesel with DPF
If you have a Diesel vehicle with DPF and require a low SAPs oil, they are also selling the 5W-30, 229.51 spec also for around £55 for 20 litres
I think there is now a new spec, which is 229.52 that caters for low SAPS as well as enhanced fuel economy. However, it seems to be that oils specs are moving to thinner oils for fuel-economy reasons, but maybe not so good for the life of the engine:
http://www.stephensservice.com/bluetec-diesel-issuesproblems/