Hi Ada
Sorry no one has replied to you yet. There are lots of factors that need to be looked at with arthritis, especially diet (I'm assuming it is Rheumatoid) mainly you are trying to aid the elimination of toxins and reduce inflammation. Obviously when your Dad is in an acute stage it would be too painful to treat with massage and a compress would be a better a better choice. There are quite a few oils to choose from, but I would probably start with just Lavender. RA can be quite painful to treat, so it is best to er on the side of caution, ask your Dad to check with his GP about treatment.
Take care xx