I think Snajeev is correctly advising. Here agents except for places like DIMOs who have their own technical influx from inhouse professional training, all iothers are using apprentices with very little knowledge and experience though under qualified persons and going to agents does not guarantee better service.
From your dialog it is apparent that there is oil getting into the exhaust system. PCV is a sure bet. Another is worn out piton rings but you can rule out by getting a compression test done. even you can do it by removing a spark plug if pertol or removing an injector then ccycle ranking with finger blocking the hole to see whether there is a plop sound when the finger is removed at compression and the cycle continues. The oils is splashed up for the cylinder to move but the scraper ring if good, prevents it antering the chamber. The other reason is as Sanjeev says, valve seats. Engine being sluggish when the light on means you have a problem related to oil entering the catalytic converter in the exhaust system. Is the engine turbo. Then the turbo valve may be stuck.
Like that systematically diagnose and attend after finding the root cause without changing componenets for the sake of repairing. Alos new spares are not as reliable as the original.
Where doyou stay? If in Nugegoda the Edirisinghe motors will do a good job but get Chulan himself involved. If in Kiribathgoda area then there is a garage before small bridge on road from Chanaka Motors to Viharaya. Nalaka Motors in Kadawatha Eksath Mawatha too is good.
