Most likely you have two separate but related issues.
Small amounts of grease and very small amounts of oil on the clutch disc will cause it to grab upon engagement. Larger amounts of oil can cause the clutch to stick after periods (even short periods) of non use. Such stickage can easily be broken but the problem will be recurring until the issue is repaired. Large amounts of oil on the disc will cause it to slip under load.
Long periods of non use can cause the clutch disc to adhere to the flywheel and pressure plate surface due to corrosion if the clutch is left engaged. Clutch stickage caused by corrosion can be much more difficult to break free but is easily prevented by blocking the clutch open during long periods of non use.
About two months ago I took delivery of a new 2012 MF tractor. The Owners Manual recommends blocking the clutch pedal down during long periods of non use and a pre-formed metal hook designed to attach the clutch pedal to the footboard so as to hold the clutch in the disengaged position is provided in the tool box.