Can an aircraft owner legally replace an oil quick drain valve?
This is what the FAR allows you to do as preventive maintenance if you are not a certified mechanic:

(c) Preventive maintenance. Preventive
maintenance is limited to the following
work, provided it does not involve complex
assembly operations:
(1) Removal, installation, and repair of
landing gear tires.
(2) Replacing elastic shock absorber cords
on landing gear.
(3) Servicing landing gear shock struts by
adding oil, air, or both.
(4) Servicing landing gear wheel bearings,
such as cleaning and greasing.
(5) Replacing defective safety wiring or
cotter keys.
(6) Lubrication not requiring disassembly
other than removal of nonstructural items
such as cover plates, cowlings, and fairings.
(7) Making simple fabric patches not requiring
rib stitching or the removal of structural
parts or control surfaces. In the case of
balloons, the making of small fabric repairs
to envelopes (as defined in, and in accordance
with, the balloon manufacturers’ instructions)
not requiring load tape repair or
replacement.
(8) Replenishing hydraulic fluid in the hydraulic
reservoir.
(9) Refinishing decorative coating of fuselage,
balloon baskets, wings tail group surfaces
(excluding balanced control surfaces),
fairings, cowlings, landing gear, cabin, or
cockpit interior when removal or disassembly
of any primary structure or operating
system is not required.
(10) Applying preservative or protective
material to components where no disassembly
of any primary structure or operating
system is involved and where such
coating is not prohibited or is not contrary
to good practices.
(11) Repairing upholstery and decorative
furnishings of the cabin, cockpit, or balloon
basket interior when the repairing does not
require disassembly of any primary structure
or operating system or interfere with an
operating system or affect the primary
structure of the aircraft.
(12) Making small simple repairs to fairings,
nonstructural cover plates, cowlings,
and small patches and reinforcements not
changing the contour so as to interfere with
proper air flow.
(13) Replacing side windows where that
work does not interfere with the structure or
any operating system such as controls, electrical
equipment, etc.
(14) Replacing safety belts.
(15) Replacing seats or seat parts with replacement
parts approved for the aircraft,
not involving disassembly of any primary
structure or operating system.
(16) Trouble shooting and repairing broken
circuits in landing light wiring circuits.
(17) Replacing bulbs, reflectors, and lenses
of position and landing lights.
(18) Replacing wheels and skis where no
weight and balance computation is involved.
removal of the propeller or disconnection of
flight controls.
(19) ...
(20) Replacing or cleaning spark plugs and
setting of spark plug gap clearance.
(21) Replacing any hose connection except
hydraulic connections.
(22) Replacing prefabricated fuel lines.
(23) Cleaning or replacing fuel and oil
strainers or filter elements.
(24) Replacing and servicing batteries.
(25) Cleaning of balloon burner pilot and
main nozzles in accordance with the balloon
manufacturer’s instructions.
(26) Replacement or adjustment of nonstructural
standard fasteners incidental to
operations.
(27) The interchange of balloon baskets and
burners on envelopes when the basket or
burner is designated as interchangeable in
the balloon type certificate data and the baskets
and burners are specifically designed for
quick removal and installation.
(28) The installations of anti-misfueling devices
to reduce the diameter of fuel tank
filler openings provided the specific device
has been made a part of the aircraft type
certificiate data by the aircraft manufacturer,
the aircraft manufacturer has provided
FAA-approved instructions for installation
of the specific device, and installation
does not involve the disassembly of the existing
tank filler opening.
(29) Removing, checking, and replacing
magnetic chip detectors.
(30) The inspection and maintenance tasks
prescribed and specifically identified as preventive
maintenance in a primary category
aircraft type certificate or supplemental
type certificate holder’s approved special inspection
and preventive maintenance program
when accomplished on a primary category
aircraft provided:
(i) They are performed by the holder of at
least a private pilot certificate issued under
part 61 who is the registered owner (including
co-owners) of the affected aircraft and
who holds a certificate of competency for the
affected aircraft (1) issued by a school approved
under § 147.21(e) of this chapter; (2)
issued by the holder of the production certificate
for that primary category aircraft
that has a special training program approved
under § 21.24 of this subchapter; or (3) issued
by another entity that has a course approved
by the Administrator; and
(ii) The inspections and maintenance tasks
are performed in accordance with instructions
contained by the special inspection and
preventive maintenance program approved
as part of the aircraft’s type design or supplemental
type design.
(31) Removing and replacing self-contained,
front instrument panel-mounted
navigation and communication devices that
employ tray-mounted connectors that connect
the unit when the unit is installed into
the instrument panel, (excluding automatic
flight control systems, transponders, and
microwave frequency distance measuring
equipment (DME)). The approved unit must
be designed to be readily and repeatedly removed
and replaced, and pertinent instructions
must be provided. Prior to the unit’s
intended use, and operational check must be
performed in accordance with the applicable
sections of part 91 of this chapter.
(32) Updating self-contained, front instrument
panel-mounted Air Traffic Control
(ATC) navigational software data bases (excluding
those of automatic flight control
systems, transponders, and microwave frequency
distance measuring equipment
(DME)) provided no disassembly of the unit
is required and pertinent instructions are
provided. Prior to the unit’s intended use, an
operational check must be performed in accordance
with applicable sections of part 91
of this chapter.