There seems to be some debate about what oil mod to choose
hopefully this will help explain the difference and make the question easier for you
Both mods are equally effective
but one is free and takes some work
and the other is easier, and costs money
Alright....
Oil Mod explained:
for future reference, I will call the Free mod, the "oil mod"
and the not free one the "oil upgrade" as it mimics Yamaha's 2007 change over.
here are some pics of mine when I did it
I did have mine torn all apart for a WR Crank upgrade, but it is NOT necessary to split the case
for the oil mod
first you need to get the side cover off (the one with the oil filter on it)
like so:
next step is to drill a hole in the filter area like this:
this hole is dead center in that filter area
and is the same size as the hole it butts up to
so just judge your drill size the same as the other hole (see second pic below)
the oil then goes out of the filter area
thru the new hole
into the case half via that existing hole
and stops at the angled flat spot at the base of the piston (now we drill this one out)

note:
this hole is up to you
I chose a fuel jet (a mikuni 87.5 jet is desired)
but you can drill a very small hole instead and that would be much less messy!
this is where you need to be very careful not to get shavings in your engine
the rest can be done with no worrys
here is a pic of mine completed
note I used an Oring to seal the 2 together (the same O-ring from the oil filter area)
some people use thin gasket material
for the oring I used a bigger drill bit and just counter bored the hole
if you want to do it without splitting the case, you would need to seal up the
area around the rod and piston real good and flush out the new hole
with a bunch of brake cleaner, and you will be fine
a couple drill bits and a couple hours and you are smiling :)
OIL UPGRADE
there are a couple places that sell this
one that comes to mind is J&T out of Lapeer MI
they take your side cover (the one with the oil filter)
and machine it a bit to fit the oil squirter into it
the kit comes with everything you need even the gaskets and a new oil pump
the new pump has higher pressure too
it is merely a bolt on kit, and aside from the down time for shipping them your side cover
and waiting for them to complete it and send it back, it is a great thing
especially for the guy that is a little skeptical about drilling 2 holes in his block!
I understand that J&T has a great reputation and is very fast on their turn around time
so I would recommend them for you if you don't mind spending the money
visit them at
http://www.jtatv.com/Yamaha_YFZ_450_Oil_Upgrade_s/4.htm
last I knew this kit costs $199 but check with the site for exact pricing
there are also places that will sell you a new 2007 side cover so you will have no down time
for like $100 more
I am here for questions if you have em
this post makes great sense to me, but I wrote it..
so please ask and I will clarify any part of it
In the interest of keeping this thread clean lets keep it on track and not fill it with "my 2004 hasn't blown up yet, and I don't have any mod" type comments