blue smoke problems

Hi all I'm new to range rovers but have worked on cars for years (maily bodywork and custom work but competant with a spanner lol).

I bought the rr with a so called manifold gasket leak that turned out to be head gasket blown between 6&8 cylinders so stripped cleaned and replaced checkin for liners slipped , piston movement and cracks but all seemed good. whilst in there I changed stem seals too and rebuilt, used 10/40 oil, with only one issue which was misfire whilst on lpg which has since been sorted as some numpty set up injector wiring for wrong banks (now sorted) also had a second hand MAF as the mesh in the other was in peices, since then it ran great for a week no issues at all but has decided to start blowing smoke around 1500-2000 rpm but only once at temp? and has used a fair bit of oil in 200 miles along with dash saying average mpg is 12mpg?

I have pulled plugs and mainly ok a couple were sooty but not oily and have read o2 voltages from ecm one is fluctuating between 2.5v - 4.5 and the other sits at 4.8v and barely seems to alter so guessing one is shot? also inlet manifold seems to be oily, but would this case smoking on both gas and petrol (gas is omvl dream and lamdas not wired into it) or could it be a combination of other things such as MAf , breathers etc

Thanks
Gary

I had a similar experience on my '97 V8. It failed an MOT for emissions and after some sound advice from Wammers it turned out to be a leaking exhaust valve. Heads off new valve everything good for 10mins. Then the blue smoke came - tons of it. Being more of a spanner than a mechanic I refused to accept the clear evidence and decided I must have done something wrong when I rebuilt the heads. I had them off two more time and replaced all the rocker gear before I accepted that problem was likely to be the oil control rings. Not a difficult job to do to be honest but needs the heads off. So off they came for a fourth time. This time success and what a difference. The oil control rings that I removed were unrecognizable from the replacements. I changed the compression rings, big end and main shells while I was down there and popped a new camshaft and followers before sewing up the top end. More or less rebuilt the engine in the end. If I had only followed the advice given from wammers, Irish Rover and Jame martin I would have same myself a lot of time and heartache.

The easy way to tell if it is the rings are worn is to do a compression test, then squeeze a couple of squirts of engine oil into the plug hole and redo the compression test. If the results are higher then your rings are shot. The other thing worth checking is that breathers from each rocker cover are completely clear.

Apologies for the delayed reply(noy had internet access) and thanks for the advice I too wondered if had done something wrong and a mechanic friend sugested that there may have been extra holes in new head gasket but checked pics I took of old against the new and couldn't see a difference so now too thinking heads off an ring replacement it is. at least I should have a fairly sweet motor when done as all the usual gremlins should be cured lol

Thanks
Gary