One more update on this

I showed this problem to a good mechanic named Shirish Joshi in Pune
After listening to my problem,he just told me to use Speed/Power/Premium petrol for one month or somewhat more or use of additives like Bardahl/Iftex with normal petrol
He told that due to premium fuels/additives in normal fuel,carbon deposited on piston rings will get cleaned and then your bike will not emit smoke.
According to this mechanic,if carbon is deposited on rings,it allows some oil to pass into the combustion chamber hence if these carbon deposits are cleaned,then no smoke.

I tried this and in 3 days itself the smoke from silencer is reduced.There is smoke for hardly 10-15 seconds now as compared to 2-3 minutes earlier.
And also since I am using castrol 20W40 oil instead of TVS Tru4 10W30,the oil level is upto the same mark even after 600 kms of oil change and riding 450 kms in one day.

I recently had a trek to Avandh-Patta fort near Nasik and I did all the travel from Pune to Bhandardara-Patta Fort base village and back to pune via Nasik on this 90,000 kms old bike and still no problem.450+ kms in one day and that too in less than 500 rupees petrol.I travelled at average speed of 60 kmph and still good mileage of around 65+
So according to that mechanic,the problem is reducing and I hope that in 1 month or more,this problem will go completely
So at last I found one mechanic who gave a good suggestion and was not desperate to open the engine.