Newbie with basic questions

[my38special]

Hello, we are the proud new owners of a 38 foot 83 Newell with a 903 Cummins. In truth, our unit needs a little TLC; however, before I tackle the "shiney" stuff, I need to look after the maintenance basics. I've looked through all the pubs that came with the bus; however, I can't find out how much oil I need for an oil change (the dipstick looks non-original and I therefore can't trust the file marks etc on it). Can anyone tell me: how much oil I should buy (including the oil filter). As well, my skeletal pub support talks about a "by-pass" filter. Could my bus have both this "bypass filter" and the normal (and obvious) spin on filter? Lastly, does the rear axle take standard hypoid 80W90?

Thanks for any help to, me, the enthusiastic newbie.