Thanks for your clear guidelines, actually I can see small amount of gunk in the front right corner of the timing cover near gasket, so may be this is start of leak, but the mileage on the my vehicle is 1,21,000 kms so may be I can go further 60,000 kms before I replace the gasket, coolant is still clean and without signs of oil contamination.
just to stretch this thread otherwise I have to start another one, the question is why mercedes benz and other automakers discontinued straight 6 engine and started V6, MB used to power its vehicles about 100 years with inline engines?