Welcome to Tractor Forum gijnpodem! As far as slopes and lean angles of operation for your tractor, there is not specific limit due to various factors such as FEL or other impliments that may effect the CG of the tractor. It is wise to ALWAYS be very conservative in this area.
I go by my seat of the pants meter (sopm). When my sopm causes me to have to change underwear, that is a good indication that I was operating on too much slope.
I suggest you very conservatively experiment on progressively steeper slopes to get a feel and idea of what is acceptable and tolerable for you and stay in your comfort zone.
Carry loads in the FEL as low to the ground as possible. Make steering and other maneuvers slowly and very deliberately. If you see that you are in trouble or too much slope either turn the tractor up slope parrallel or down slope to regain control.
Be sure you have more than enough counter weight on the rear of the tractor to ensure you get good solid traction.
I am sure some others will jump in with some good tips and advice as well.