The only way to add oil is when you re-gas. PAG oil to be specific. The oil does degrade over time as do all lubricants. AC service is manufacturer recommendation as is cambelt - nothing to do with what a dealer or independent garage decides.
I work in a motor factor, firstly the HO1234YF gas is massively expensive with dealer stealers charging £300 upwards for a regas. This gas is in all cars 2015 onwards and only about 1 in 10 independent garages have the equipment to do this, next couple of years they will be able to charge...