I would recommend pump oil for this particular application because it contains special components in the oil but you can substitute as I have done many times with hydraulic fluid.You need to change your pump oil every 50 hours.
Question edited for a lot more clarity, add maker, fix typos, change into what I think you were saying. Manuel is the name of a Spanish gentleman, 'manual' is the word you need.
Manuals in the link, plus the maker. This link show 'M Series' belt.
https://www.manualslib.com/manual/1865101/Lawn-Boy-M-Series.html?page=153
Lawnboy
https://www.lawnboy.com > customer-support
If you need to find a Lawn-Boy retailer, have a question about your mower or want to learn more about its warranty, Lawn-Boy's customer support has you ...
..