I own 4 bauer compressors and would have nothing but a bauer. I guess my statement came off wrong. But with synthetic oil and a filter what contaminants could you get.
If all is working well, you should get nothing through. However consider this, no pressure maintaining valve (PMV) means the air velocities are such that a normal healthy filter is trashed within 5-10 min in particular when filling a very empty cylinder. Thus you get water and oil (all be it synthetic) into the cylinder. Neither which is good to breath. that the oil is synthetic still doesn't make it OK to have significant amounts in the cylinder. The reason for using synthetic is that its flash point is higher, operating temperature range higher, less carbon thus less contamination, longer run times, lower risk when mixing gases etc etc. However the oil is still bad to allow into cylinders regardless of its composition. Along with this the assumption is that the filter is of a good design and reasonable size such that it can fill quite a number of cylinders before requiring replacement.
In the instance of my chinese special, I had the combination of no PMV and a filter tube about 19 mm diameter and perhaps 120 mm in length. The volume of filter material was small (by comparison to a P21 or 0520 cartridge) thus had it fitted into the filter body correctly (which it did not) and if the compressor was fitted with a PMV (which it wasn't) I suspect maybe 5-7 tanks and it would require a filter refill. Not a long time at all.
Given however that the filter tube did not seal in the filter body correctly the air was bypassing the filter material, with no PMV this added to trashing the filter material anyway, and with so little filter material was a joke as well. The effect of using it on my first 8 tanks was to contaminate all 8 tanks to the point that the steel tanks (all brand new) began to rust inside and had a coating of oil inside tank and valves. All the aluminium tanks had water and oil contamination. Remember this is the "first" 8 tanks.
By comparison, my bauer filled about 15 or so tanks ( I think about 37,000 litres by volume) before I changed the first P21 filter and all with "clean, uncontaminated" air.
Just because a compressor uses synthetic oil and a quality filter doesn't guarantee a good result. if the compressor has a lot of blow by thus pumping significant volumes of oil with the air, the filter (all be it a quality one) will quickly be consumed and trashed. If you are not checking the results you can easily pump contaminated air.
I my case I have double filtering. A P21 Bauer in series with a 0520 in line cartridge. I check the weight of the second in line cartridge regularly and this indicates if the P21 is exhausted as if the 0520 starts to increase in weight (thus showing that the P21 is exhausted and passing contaminants). Although I change the P21 before it gets to this stage anyway. It also ensures that water and oil is removed fully. It might be overkill but I like my air clean and I like staying alive by minimising risks where I can.
The main point being just because you use quality synthetic oil and a quality filter cartridge, doe NOT guarantee clean air, there are many other things to consider such as status of the compressor rings/cylinders, PMV operation, filter issues allowing air bypassing, poor manual draining by the operator flooding the filter element etc.
