You can put less than a full can into the system just the same way you would put only some coolant ,, by weighing it while feeding it through the valve and stopping when it gets to the proper weight....

Did I mention you can buy pressurized cans of oil ? Just like freon cans... same top on them so you can use the same lines and fittings....

However, some types of compressors also have a resorvoir and use a dip stick to check for the correct level... you have to make your own dip stick and they give exact bend and radius for what you need.... If you want to guess how much your putting in you can just stick it into the filler hole on the side... there are two for Yorks for instance because it can be mounted in either of two positions...