Hi and welcome to the forum
Need to go here, just enter you model number:
That bring another screen and then you will have to search around for that info. There should find an on line manual that will tell you what you need.
My guess the other tank is for the Hydro drive oil system, it to should be covered in the owners manual on how often it should be changed.
If prior owner has not changed in that many years, I would put a few hours on it and then change it again. The new oil and filter, if it has one will get pretty dirty quickly since the new oil will doing a lot cleaning.
I use a straight weight 30W in my mowers and change it once a year, or about every 50 hours. Just because you change the oil, don't forget to check everything before you use the mower!