Air dryer purging oil, is this normal?

It's supposed to purge oil from a coalescing air dryer, it protects the air system... That's good

 

A Bendix dryer contains a desiccant cartridge. The purpose of the desiccant is to absorb moisture and then expel it when the dryer purges. The desiccant will absorb oil as well and once its fully contaminated with oil it can no longer absorb water so the dryer is no longer able to do its job. Its normal for an air compressor to pass a small amount of oil. If the dryer is expelling a lot of oil your compressor needs to be rebuilt or replaced.

 

I am a living testament to the absolute need to have a air dryer to protect your rig's tanks from water and oil and anything else liquid. It's not good to get into that story because Ive already written about it a few thousand posts back.

I have the assumption that if the air dryer starts (Usually Bendix) to throw down oil I'll have the shop take a look. It hurts nothing for someone to do so.

 

There is oil from your air dryer because the air compressor is passing oil. It is not suppose to do that. Oil very quickly destroys the desiccant material inside the filter. Air compressors passing oil is a sign of either a restricted air intake, restricted oil return or simply worn out, or a combination.

 

The small air leaks can also cause the compressor to cycle much more than you expect.
Every time it cycles it is pumping more oil into the system.

 

That really does not sound bad. The only truck I drive for any period of time any more was made in 89. The compressor cuts in no more than once every 30 minutes. It is spring ride. Many variables with air ride plus stop and go driving. The point is if a leak causes your compressor to run twice as much as necessary it doubles your air drier problem.
Back to what Heavyd and Inderjit said.