Pros
Good initial salary, 'sarkari' tag (to flaunt off in your village), job security, great exposure and opportunities for skill development, lots and lots and lots of benefits and compensations, unlimited medical cover for self and dependants even after retirement, good friend circle, opportunity to roam around India (with subsequent transfers) whether you like or not. Recently (from past 3 years) IOCL has stopped recruiting for non-officer posts which is the beginning of the end of Union. So new comers will have less to face such dirty dealings.
Cons
Narrow minded people in the upper management, initial salary is good and perhaps better than your friends but very soon you will see them outrun you in terms of salary and even after 20 years of service you will have a decent car staying in a rented house having only enough bank balance to keep you and your family alive (of course I am assuming you won't indulge in malpractices, the scope of which is quite limited as IOCL policies are good to check such happenings), monotonous work and hardly it will be related to what you studied (work really doesn't require an engineer), you will sometimes find chiefs (who are generally 50-55 years of age) doing the same job as that of a fresher, frustrating work life if you have to work in shifts (and many times even if you don't have to work in shifts), limited approval for holiday requests, interacting with union members (let me warn you, it more often than not, turns out nasty, not because you are bad at handling such delicate issues, but because union members love being and have always been pampered by company higher officers and also they are quite egoistic). Be ready to put your hands in the dirt. You will find some employees (say, one in three) doing 20% of the work that you do and taking home 200-300% of the salary that you get and you will have to do their part of the work which will sometimes boil your blood. Also be ready to put your self-esteem in the dustbin during working hours. After few years into the service you will have to continue this job just because it will be quite difficult for you to find new (private) job where this work experience will matter.