Say the oil and filter has been changed just a couple weeks prior to the annual. Does an annual inspection always have to part of the annual? Or can it be omitted?
Or is this possibly type-specific / maintenance manual-specific?
As far as I know the FARs do not require oil filter inspection as part of A&P IA annual inspection of the engine, and it would be pointless with few hours since filter was changed. Maintenance is otherwise unrelated to his inspection per the FARs, although it may be a convenience for the owner to do it at the same place and period of downtime.
It is no different than it would be in relation to a TÜV inspection on a car.
For an N registered aircraft, the following guidance is provided:
Title 14: Aeronautics and Space
PART 43—MAINTENANCE, PREVENTIVE MAINTENANCE, REBUILDING, AND ALTERATION
Appendix D to Part 43—Scope and Detail of Items (as Applicable to the Particular Aircraft) To Be Included in Annual and 100-Hour Inspections
(a) Each person performing an annual or 100-hour inspection shall, before that inspection, remove or open all necessary inspection plates, access doors, fairing, and cowling. He shall thoroughly clean the aircraft and aircraft engine.
(b) Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the fuselage and hull group:
(1) Fabric and skin—for deterioration, distortion, other evidence of failure, and defective or insecure attachment of fittings.
(2) Systems and components—for improper installation, apparent defects, and unsatisfactory operation.
(3) Envelope, gas bags, ballast tanks, and related parts—for poor condition.
(c) Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the cabin and cockpit group:
(1) Generally—for uncleanliness and loose equipment that might foul the controls.
(2) Seats and safety belts—for poor condition and apparent defects.
(3) Windows and windshields—for deterioration and breakage.
(4) Instruments—for poor condition, mounting, marking, and (where practicable) improper operation.
(5) Flight and engine controls—for improper installation and improper operation.
(6) Batteries—for improper installation and improper charge.
(7) All systems—for improper installation, poor general condition, apparent and obvious defects, and insecurity of attachment.
(d) Each person performing an annual or 100-hour inspection shall inspect (where applicable) components of the engine and nacelle group as follows:
(1) Engine section—for visual evidence of excessive oil, fuel, or hydraulic leaks, and sources of such leaks.
(2) Studs and nuts—for improper torquing and obvious defects.
(3) Internal engine—for cylinder compression and for metal particles or foreign matter on screens and sump drain plugs. If there is weak cylinder compression, for improper internal condition and improper internal tolerances.
(4) Engine mount—for cracks, looseness of mounting, and looseness of engine to mount.
(5) Flexible vibration dampeners—for poor condition and deterioration.
(6) Engine controls—for defects, improper travel, and improper safetying.
(7) Lines, hoses, and clamps—for leaks, improper condition and looseness.
(8) Exhaust stacks—for cracks, defects, and improper attachment.
(9) Accessories—for apparent defects in security of mounting.
(10) All systems—for improper installation, poor general condition, defects, and insecure attachment.
(11) Cowling—for cracks, and defects.
(e) Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the landing gear group:
(1) All units—for poor condition and insecurity of attachment.
(2) Shock absorbing devices—for improper oleo fluid level.
(3) Linkages, trusses, and members—for undue or excessive wear fatigue, and distortion.
(4) Retracting and locking mechanism—for improper operation.
(5) Hydraulic lines—for leakage.
(6) Electrical system—for chafing and improper operation of switches.
(7) Wheels—for cracks, defects, and condition of bearings.
(8) Tires—for wear and cuts.
(9) Brakes—for improper adjustment.
(10) Floats and skis—for insecure attachment and obvious or apparent defects.
(f) Each person performing an annual or 100-hour inspection shall inspect (where applicable) all components of the wing and center section assembly for poor general condition, fabric or skin deterioration, distortion, evidence of failure, and insecurity of attachment.
(g) Each person performing an annual or 100-hour inspection shall inspect (where applicable) all components and systems that make up the complete empennage assembly for poor general condition, fabric or skin deterioration, distortion, evidence of failure, insecure attachment, improper component installation, and improper component operation.
(h) Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the propeller group:
(1) Propeller assembly—for cracks, nicks, binds, and oil leakage.
(2) Bolts—for improper torquing and lack of safetying.
(3) Anti-icing devices—for improper operations and obvious defects.
(4) Control mechanisms—for improper operation, insecure mounting, and restricted travel.
(i) Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the radio group:
(1) Radio and electronic equipment—for improper installation and insecure mounting.
(2) Wiring and conduits—for improper routing, insecure mounting, and obvious defects.
(3) Bonding and shielding—for improper installation and poor condition.
(4) Antenna including trailing antenna—for poor condition, insecure mounting, and improper operation.
(j) Each person performing an annual or 100-hour inspection shall inspect (where applicable) each installed miscellaneous item that is not otherwise covered by this listing for improper installation and improper operation.
I’ve posted that list more times than I can count but nobody ever believes it 
An FAA regulated annual inspection is indeed a simple thing.
Item (d) (3) requiring screen inspection could be manipulated into an interpretation requiring oil filter inspection, but I doubt very much whether that occurred to the authors. A screen inspection is not the same as cutting apart a filter, and nobody thinks oil filter cutting/inspection is a legally mandatory maintenance (versus inspection) item either.
Note also that the method to check cylinder compression is not defined. Pulling it through by hand is one method 
That’s not relevant to the question but helps one to understand that the environment anticipated for an FAA Annual includes a dirt floor barn inspection of a crop duster taking a one day pause from work, etc.
“(3) Internal engine—for cylinder compression and for metal particles or foreign matter on screens and sump drain plugs. If there is weak cylinder compression, for improper internal condition and improper internal tolerances.”
Check sump drain plug for metal, without draining oil???
The issue is not draining oil, it is the lack of obligation to replace it when it is almost brand new. Obviously you can put the same oil back in, if you like.
It’s not legally mandatory; I don’t change mine if it has say 10hrs on it at the Annual.
A number of SR22 pilots had been going to 100hrs between oil changes because the book says you can, so probably just did it at the Annual, but the oil is in a terrible condition by then.
Not if you use unleaded fuel. After a few changes I’m certain it stays cleaner longer.
Not sure if a SR20/22 can run on UL91 mind.
