They must be Oil Free type dental air compressors, meaning no oil anywhere.

I don't think there is a good answer to your question relating to Dental air but there is a good answer relating to Medical Air which I would reference to your concern of hydrocarbons getting into the pipeline. 

If 2021 edition 5.1.3.6.3.4 (A) (2) and (3) are met relating to Compressors for Medical Air are met than, I would argue that 5.1.3.6.3.12 (E) (2) and (4) should be followed. 

5.1.3.6.3.12 Operating Alarms and Local Signals. Medical air
systems shall be monitored for conditions that can affect air
quality during use or in the event of failure, based on the type
of compressor(s) used in the system.

(E) Where compressors compliant with 5.1.3.6.3.4(A) (2) and
5.1.3.6.3.4(A) (3) are used, the following requirements shall
apply:

(2) Coalescing filters with element change indicator shall be
provided.

(4) Gaseous hydrocarbons shall be monitored on a quarterly
basis.

The reason I would argue this is because the concerns mentioned of dental air should not exceed the same concerns of Medical Air in my opinion. We should agree that Medical Air needs to be the higher standard in this case. If we are meeting a higher standard, than it is acceptable under these conditions.

Merry Christmas everybody!!

Most interesting discussion!   Reminiscent of the arguments back in the day around what compressors should be permitted for medical air!

Al is right - oil free means "no oil in the product air", but not "no oil in the compressor".  So, in code terms (2021 99)

5.1.3.6.4 (A) (1) compressors: Oil less (sealed bearings) {scroll, oil less reciprocating}; 

5.1.3.6.4 (A) (2) compressors: Oil free reciprocating (oil in gearend but not in airend) {T-30 ish, but the T-30 itself doesn't cut it} 

5.1.3.6.4 (A) (3) compressors: screw/tooth compressors (oil in gearend but not in airend) {Z compressors, oil free screws}

5.1.3.6.4 (B) compressors: liquid ring.

To me, the dental rule as written would appear to allow any of the first three. 

I disagree slightly on the cleanliness requirement - I think dental air may be slightly more critical because as I understand it a tiny amount of oil will interfere with the bonding process, so oil free air means a very great deal to the dentist from a practical viewpoint. Maybe Dan Shoemaker can weigh in?