To begin, HFCs (134a and 404a / 507) MUST have some of the mineral oil replaced with polyolester (POE). Most manufacturers recommend less than 5% residual mineral oil, although that is an arbitrary number and recent studies have show larger amounts of residual mineral oil will still work fine. Some blends contain hydrocarbons, which help with mineral oil circulation even though the HFC blend will not mix with the MO. Larger, more complicated systems will still require the addition of POE to help mix the oil with liquid refrigerant, for example in a receiver.
With the older R-12 and R-502 retrofit blends (401A, 401B, 402A, 402B, 408A, 409A, and similar blends), replacement of some of the mineral oil with alkylbenzene (AB) or POE is recommended at lower temperatures. Evaporators running above 0ºF will generally be able to return mineral oil with these blends.