Firstly yes I've read the manual and know the manual says 1.1L with filter, which is what I have been doing.
The FAQ in the WR/YZ 250 forum states this:
3) 01-02 owners should check the oil AFTER it has been started and the engine warmed up. then wait a few seconds. the manual clearly goes over the correct procedure for this. 01-02 bikes can hold 1.6L of oil with a filter change. the 03 holds 1.1L with filter change (but 03-05 can hold just as much as 01). the 06 holds 1.25L with a filter change. the 07 holds 1.15L with filter change
I vaguely recall reading a post about this somewhere but can't find it now - is this is also true for a 2005 WR450?
If so is there any reason I shouldn't add that extra half litre for general trail riding (open desert tracks mostly)? It would seem it should give longer oil life, so I could extend oil change intervals for multi-day trail rides, or maintain better quality oil for normal weekend riding with normal oil change intervals. I buy Yamalube synthetic from a bulk drum at dealer cost so I'm ok with the increased service cost.