If the cooling is SUFFICIENT and there are bubbles present, then SOME bubbles are permissible. But that is only when the ambient temperatures are VERY high. But if bubbles are present in the sight glass when the ambient temperatures are at the current temperatures in Lahore, then it definitely does indicate that the R134a refrigerant gas is insufficient. And since the majority of the "backyard and bonehead" mechanics, as well as the Toyota and other car dealers in Lahore are NOT charging AC system by weight, which they should be in the first place, this is exactly why you CANNOT charge a R134a system using the sight glass method, e.g., as soon as the bubbles disappear, then they would say that the refrigerant charge is full. That is TOTALLY wrong. That method is for R12, NOT R134a. So all the mechanics are STILL keeping in mind the old lessons taught before 1994. I have done several car AC service, for my close friends and just other people who need help in general, and I use a proper digital scale to weight the exact amount of refrigerant gas, and not only are there no bubbles, but the AC system has the full charge, the compressor does NOT overheat and trips the way it should, the refrigerant is circulating compressor oil entirely in the system and they are good to go.
That's right, you cannot mix R12 and R134a because the chemistry of the refrigerant gasses are different, and the oil types are different. POE is the type of oil that was in R12 and PAG is the type of oil that needs to be used along with R134a. If you mix R12 with R134a, it will not be compatible with the type of rubber the O-rings around all your joints are fitted with, which are also different for R12 and R134a. Add to the fact that if you mix R12 with R134a, could cause the gasses to actually create hydrochloric acid in the AC system and cause some of the components to corrode from the inside, I definitely would not advise anyone to do that. A lot of the world will be changing to HFC-152a and HFO-1234yf, as the Europeans have mandated to phase out R134a by 2017-2018, because it is still a Greenhouse emitter.