T3331400 was superseded by T3331520 so
T3331520 is the one to use.
I don't know if there's much difference in the two parts,
but the later one is likely to be a little more "durable" !

If neither Part is available, there is another alternative in T3331614
which has a "Grease Groove" incorporated into the shank,
and while some Members
have used it, it has never
been officially specified for use on the Trophy.
With regards to your other questions :
1.
Bike Bandit or
Hermy'sAlso have a look on the Forum here for Parts details :
Trophy Spare Parts List2. Plugs are only scheduled to be "Checked" at 10,000 miles, and replaced at 20,000.
If you're taking them out to "check" them, then for the comparatively small extra cost,
it's probably worth replacing them. OEM = NGK CR8EK.
You can get Iridium Plugs ( NGK CR8EIX ), but I can't see any benefit for the extra cost.
3. Semi or Fully Synthetic Oil to 10W/40 or 10W50 Grade.
Recommended brands include Castrol Power 1 Racing 4T, Silkolene Pro 4,
and many others - depending on who is doing the recommending !
Cheers

