As long as the plunger makes contact with the fluid, it is ok. If you over fill the damper the consequence is that it will spirt out (until the proper amount is left) and dirty the engine compartment. Hope this helps
I have a 73 240z with the flat top carbs. I have experienced the same problem. I found that there is a product on the shelf called 3 in 1 Oil. You may have heard of it before. If you read the print below the 3-1 product name it says that its 20 weight oil. So I use that... haven't had a problem with it yet...
How full.... there is a line on the plunger rod. However this isn't a big help. Because when you wipe the plunger down or dry it off so to say... the rod becomes shiny and when you place it back in the carb to see where your level is at, you will be unable to see the mark that the wet oil is making. So before anyone makes any old man jokes... I have 20/20 vision and don't wear glasses or contacts... but I'll be darn if I can see the mark. So what I do is this. I fill the tube up until its about 3/8 to a 1/4 inch from the top. And I never have had a problem.
Side comment... the flat top "bowls" come off by removing the 3 screws that hold them on. once removed there is a spring and a "plate" that lift right off. The plate holds the oil. I have removed these parts, cleaned them with carb cleaner and put them back with no changes required on the carb.
Lastly, just about everyone here will advise to "trash" the flat tops... even tho they perform ok... the better setup is with the SU's. I have rebuilt my flat tops and find that getting them to ide as nice as the SU's is impossible. The SU's also are noted for giving better fuel mileage... but I really think that depends on your foot. I looked high and low for SU's and have found them with an earlier air cleaner for $150.00.