1. I think that you might be facing the same situation which i faced last year.
I went to Mr Attahr mechanic at islamabad for tunning which he did and car started emitting a little white smoke. He gave me his fatwa that engine is week. i told him that this engine was overhauled just a few months back so how this engine can be week now. Then his son gave me another idea to check the oil quality. i was using caltex at that time.
At the same spot i met a pakwheeler Saqib who gave me detailed info about oil grading number and its qualities.
After coming back to lahore i made a research on the availabel oils and started using kixx engine oil 10w40 and smoke vanished. Means these shell/hell and caltex companies and are selling v. bad oil at the price of nice oil grades. Particularly shell is a hell for me (for mineral oils).
now again i have shifted to petmin 10w40 and its really marvellous (5000km is nothing for it) . frequent shifting of engine oils is also not good for engine, but sometimes we have to do it to get the best one in economical range.
2. Same like you, i got suspicion that car is reducing oil as oil gauge was showing very varying oil levels at diff times. So i asked my mechanic to get out all engine oil in a clean dish and check its quantity.
Quantiity was all ok and actaully the gauge was culprit.
So you can check your problem this way also, if you like.