Does it get logged ?  What's acceptable before you get carbon Monoxide poisioning ?

We don't have a CO detector in the cabin so; there's no oil being burned.

Oil consumption Limits are in the engine maintenance manual. Adding oil is considered servicing, not maintenance. Unless there is a exhaust leak into the cabin, CO issues unlikely...

Let's move on to the next random thought.

see Appendix A to FAR 43(c)(6)

It's servicing...

b. Preventive Maintenance. Section 1.1, defines preventive maintenance as “simple or minor preservation operations and the replacement of small standard parts not involving complex assembly operations.”

Do you log fuel added in the maintenance log?

Your arguement is "do you log adding fuel"?   Fuel is not lubricant!  Oil is a lubricant.  Lubricants are maintained to prevent engine failure.  If the engine oil is low, it the in an airworthy condition??  What is the planned consumption rate of engine oil in a flight?  Do you have to report it on your flight plan "Oil Endurance in Hours and Minutes"?   No, because it is not expected to be consumed, like fuel is!  

You gave the 1.1 general definition, but Appendix A, (c)(6) specifically says "lubrication" is a preventative maintenance.  

If the transmission oil is low, do you have to write it up?   T/R gearbox?  Rotor Brake fluid??

If it's not preventative maintenance, you are saying ANYONE can do it, including the 16 kid who drives up on his skateboard.   "Hey kid, fill up that transmission for me, will ya?"

Maybe in a Part 91 aircraft, but this is a for-hire commerical operation under Part 135, which prohibits preventative maintenance from being performed by just anyone.  If it is not even preventative maintenance, such person can service your commerical operation aircraft.

If your engine has a oil consumption limitation, the ONLY way you can show you meet that is by documenting the oil you add to the system each and every time.   I have yet to see a fuel consumption limitation.

If you are adding oil more often than normal, write it up as a descrepancy..

by another person if it is not documented?

If what you are smoking is turbine oil,  get a CO2 detector. 

to add oil and you have to make a logbook entry.

W T F?

You're right, pole vaulting over a mouse turd

AFM: servicing

Sounds like we have a non standard method of doing business.  "Servicing or Maintenance"  Call it what you want ?   It doesn't matter what you call it.  Please log it.

My mechanic was really good at adding oil.  I never saw any write up, on any amount of oil added.  He probably added a half a quart every four hours in a 407.    Every time I ran the heater, I could smell the smoke, with the carcinogens in the synthetic oil, I was inhaling a boatload of poison.

I'm talking to the largest ems operator in the States, you safety types, Please put a CO detector in the cabin for me. Thanks . 

I drink. I just drink till the bottle is all gone.

Link to just one of your several FAA references on this topic, please?

see link to FAA legal interp below which stares:

The subject of adding oil is often debated; however, we note that it is not an item included in pa1t 43, appendix A, paragraph (c), which lists items the FAA considers to be preventive maintenance. Accordingly, while it may be prudent to include in the maintenance log the adding of oil, including the quantity added, doing so is not required by the regulations 

https://www.faa.gov/sites/faa.gov/files/faa_migrate/interps/2016/Hochberg_2016_Legal_Interpretation.pdf

... "servicing" is a maintenance function.   When the FAA addresses those matters, then their interp has some real merit.   However, this is limited to Part 91, as stated in the letter. 

Part 135 9 or less passenger operations follows part 91 and part 43 for maintenance records requirements..

Part 135 operators who have aircraft with seating of 9 or less must also follow this FAR, which describes maintenance requirements.  Note, it's maintenance to follow the "maintenance program" for an engine.  So, if the manufacturers recommended maintenance program tells you how to maintain the engine oil, it is a maintenance function.   Maintenance must be documented IAW FAR 43!

§ 135.421 Additional maintenance requirements.

FAA has recognized "servicing" is maintenance!  But, they failed to address that in the letter you cited (likely, the attorneys are not that familiar with their maintenance technician's manuals). 

The FAA came up with the list of Preventative Maintenance so that Pilots and aircraft owners can perform minor maintenance on aircraft being operated under Part 91.  Pilots cannot perform any Preventative Maintenance on aircraft being operated under Part 135 unless specifically authorized in their OPSPECs.  That where you'll find HAA pilots specifically able to service the oxygen/LOX, install/remove named equipment, add oil to engines, etc.  If it's not in your OPSPECs, it isn't authorized.   But, it is still a maintenance function!

In the Aviation Maintenance Technician Handbook - General, pg 1-20

Servicing Aircraft
Servicing Aircraft Air/Nitrogen Oil & Fluids
Checking or servicing aircraft fluids is an important
maintenance function. Before servicing any aircraft, consult
the specific aircraft maintenance manual to determine the
proper type of servicing equipment and procedures. In
general, aircraft engine oil is checked with a dipstick or a sight
gauge. In general, aircraft engine oil is checked with a dipstick or a sight
gauge. There are markings on the stick or around the sight
gauge to determine the correct level. Reciprocating engines
are to be checked after the engine has been inactive, while
the turbine engine must be checked just after shutdown. Dry
sump oil systems tend to hide oil that has seeped from the oil
tank into the gearcase of the engine. This oil does not show up
on the dipstick until the engine has been started or motored.
If serviced before this oil is pumped back into the tank, the
engine overfills. Never overfill the oil tank. Oil foams as it
is circulated through the engine. The expansion space in the
oil tank allows for this foaming (oil mixing with air). Also
the correct type of oil must be used for the appropriate engine
being serviced.

How is one to know if the correct type of oil was used for the engine being serviced if it is not documented?  That's why oil is a maintenance action and falls under the lubrication listing of what Preventative Maintenance is!    Fuel, on the contrary, is not a lubrication or maintenance function!

Pilot's Handbook of Aeronatuical Knowledge, pg 7-16

Oil Systems

The engine oil system performs several important functions:
• Lubrication of the engine’s moving parts
• Cooling of the engine by reducing friction
• Removing heat from the cylinders
• Providing a seal between the cylinder walls and pistons
• Carrying away contaminants

Appendix A to FAR 43:

(c) Preventive maintenance. Preventive maintenance is limited to the following work, provided it does not involve complex assembly operations:

https://www.faasafety.gov/files/notices/2018/Aug/First_Issue.pdf

Does your aircraft have a set of Manufacturer's Maintenance Manual and/or Instructions for Continued Airworthiness (ICAs) that tell the maintenance folks how to "service" the aircraft oil? 

A manufacturer is required under §21.50(b)
to prepare a complete set of ICAs in
accordance with (IAW) the applicable certification
standard for the product (14 CFR parts 23, 25, 27,
29, 33, 35) that is acceptable to the Federal
Aviation Administration (FAA). The purpose of the
ICAs are to enable persons authorized by the FAA
to maintain the continued airworthiness of the
product and approve the product for return to
service. The manufacturer is also required to
furnish the ICA to each owner of the product and
then make it available to persons requiring its use.
This is how the certification rules interface with the
continued airworthiness rules of part 43 and 14
CFR part 91. In addition, as part of the ICAs, the
manufacturer is required to provide airworthiness
limitations (AWL). The FAA-approved Airworthiness
Limitation Section (ALS) is required to be separate
and distinct from the remainder of the FAA-
accepted ICA document. All of the certification
standards (e.g., parts 23, 25, 33, 35) require ICAs to
have an ALS that states the following: “The
Airworthiness Limitations section is FAA approved
and specifies maintenance required under §§ 43.16
and 91.403 of Title 14 of the Code of Federal
Regulations unless an alternative program has
been FAA approved.”

If you don't record rate of consumption, then you should never have to replace the engine for having a faulty oil seal.

The idea is to keep from breathing carbon monoxide when the heater is on.  If you don't record usage, how will you know the seal is bad ?

needs to be documented!  It is a lubrication!