Is Mobil 1 ok for 83 Tenere
 

The mechanic told me only to use it from new on a bike. Replacing normal oil with synthetic will do damage. Does he know what hes talking about? If so whats the best oil for longevity?

mineral oil will be all right, not necessary to spend so much money on synthetic oil.
yamaha has specified SE grade to be used in xt600 engine. almost every oil you can buy nowadays even in supermarkets is better than SE (SE -> 'poor' quality, ..., SF -> good quality).
much more important for long engine life is regular oil change and proper treatment of engine.

regards,
klaus