AC DIY recharge

yes in a commercial system the oil does sit in the compressor but in auto ac it is in with the gas, I have had numerous new vehicles from several different manufacturers and since my first taste of aircon in 1983 I have had it in every vehicle since and generally at the 3 years + mark the cooling efficiency reduces and only perks up after re-gassing, the 6 to 8% depletion quoted are the figures given to me by couple of independent auto ac companies I have used over the years, I remember one of the company Astra vans being sat for a day under test pumped up and full of dye checking for a leak and they couldn't find anything amiss but the dam thing still had to be re-gassed every 2 years to keep the ac ok, and as several other members have already pointed out in frequent use does more harm than good