Hi Peter.

R-12 was the best with oil return. Never had any problems.
R-22 is good but not as good, R-404 is new and now you know that it has problems with oil, but maybe pipes diameter should be changed or the pipes in the evaporator should be arrange different.

I can see that the industry is still struggling with the new refrigerants.

Its something you will have to find out by yourself, very slow.

Then you will be an expert on "oil return in refrigeration systems operating with R-404A"

Chemi