We have used an overloaded V8 127 for the last 58k miles – down to W.Africa a few times and a couple of trips to Morocco and RIM. At 3.25 tons plus, we can get an average of around 22 to 23 mpg with a light foot on tarmac or graded roads but this goes down to about 8mpg in sand.
The engine is great as long as you look after it – we change the oil and filter every 6k, but as suggested you could do it more frequently. I also use change the plugs, dizzy cap and rotor arm every 8k which seems to keep things running sweetly and have had the carbs balanced professionally a couple of times. When driving in sand in low / diff lock, use a soft foot and change gear slowly otherwise the transfer box and UJ’s will take a real beating with all the torque from the engine.
Driving any real distance on soft sand in the low box will lead to overheating so keep an eye on the temp gauge and stop every hour or so for 15 min with the bonnet up and the engine running to cool it down if it gets near the red. As suggested make sure the visco unit on the fan is in good shape – I have replaced 2 now that seem to have become a bit slow to react and I usually take the old one as a spare.
As for the gearbox – its very agricultural but enormously strong. The bearings are the only weak point but I have driven back from RIM to the UK with the gearbox sounding like its on its last legs and it still kept going. I only fixed it as the noise was unbearable! The noise starts in all gears apart from 4 and reverse (lay shaft bearings go first – then the main) and gets louder and louder until it sounds like your dragging sheets of corrugated tin around with you. Tip -If this happens, empty the gearbox oil every 1k or so, filter it through a t shirt to get the lumps out and put it back in again if you can’t get any new stuff. I got another 8k out of what I had decided was a completely knackered box this way. I have also been told that 20/50 engine can be used and was recommended for older boxes.
I like the tip of building a heat shield to keep the cab cooler – we got it up to an uncomfortable 60 -65 deg a few times and kept stopping and standing in the sun to cool down!
Hope this helps – have fun!