"What will cause zinc levels in my oil to fluctuate?"
There are several reasons why zinc may be present in your oil samples. These include wear of zinc-containing alloys such as brass, zinc plating and galvanized surfaces, as well as zinc-containing additives, most notably zinc dialkyldithiophosphate (ZDDP), which is a common anti-wear and antioxidant additive.
The most common reason for fluctuating data is improper sampling. Sampling accuracy depends on sampling from the correct location and using the same methods and procedures each and every time. If you do not have a good handle on the correct procedure, this would be the first place to start looking.
Other factors that may impact zinc concentrations include the addition of make-up oil (possibly the wrong oil) and the method used by the lab to analyze the sample. This is particularly true if the data is being compared between different labs that may use different instruments. Changes in particle size if the source of the zinc is from active machine wear can also affect zinc concentrations.