If the forks are marked ‘All Travel’ then you can adjust the travel by adding or removing plastic spacers.
I have used engine oil when I needed to do a lower leg service in Morzine one year. Worked fine and the forks felt great. That said, however, ‘proper’ motorbike fork oil can be got from Halfords and alike cheaper than the mtb specific stuff.
Check Srams website for the user manual for your forks and that’ll contain the details on what you need to do. There are also numerous You Tube how to vids. Also ‘Enduro Seals’ produced some very good and easy to follow guides for swapping over to their seals.