An easy check for bad coils. Remove coil cover, wait til dark, make sure engine is warm, watch coils with engine running in a nice dark area and if your coils are arcing out you will see nice blue lightning around the base of the suspect coils.
Oil is perhaps a bit on the thick side. Nissan spec 7.5-30, I personally use 5-40 and it seems to like it.
Fuel economy, how heavy is your right foot? Are your o2 sensors working correctly? Are you pushing more than stock boost? A lot of factors affect economy. Just before mine came off the road I was getting ~220km to 45l, driving like a grandpa, rich as a pig.
Hope some of this helps.