Last sept I paid for a recon vege DG1900 to replace the knackered 150K one in my van. Not the cheapest replacement but felt the extra cost would be worth it in the long run. It's got a 12 month warranty.
Literally everything behind the gearbox was replaced, including lots of new hoses, corrogated airways, leads, coil, dizzy, etc, etc.
It's developed a minor problem and the fitter's have asked me to bring in.
Over the phone they gave me a few explanations what might of happened and want to do a compression check next tuesday. The fault could be sticking piston rings on the right bank.?
Basically, both plugs

My questions are.....
What causes or fails and lets oils on the plugs?
Will a compression check show upl the problem?
If the RHD cylinder head is replaced should I negotiate an extended warranty?
What else should I ask next tuesday? No rude replies please

I know these are basic questions. I'm not mechanically minded and want to go back pre-armed so to speak.