Also, if it not an item listed as Preventive Maintenance, then it revert to  

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.”