Poor fuel economy - where to start?

[solitaire]

Well I would say ive got about 40km's left in the tank and have managed 715km's so far, so mileage is definitely improved.

Things I had changed:

Leads
Fuel Filter
Fuel cap (had a hole in it!)
Cleaned MAF
Reoiled k&N, ran car for 500kms and re-cleaned maf
checked tyre pressure and it was already ok

so not really sure what fixed it

I did previously get more than 700k's out of a tank with a hole in the fuel cap and pretty much everything else above so I am really quite confused.

[solitaire]

Filled up today and it worked out to 10.5L/100kms which I am extremely happy with for a v8. Will make sure its not just a anomoly by check the next few fill ups, but definitely better.

[RUKI]

Awesome suggestions, thanks - found that denso sell universal o2 sensors that will work but its all ex usa with insane postal charges... anyone on here a denso reseller? don't want hookups, just reasonable retail...

I've ordered my Denso (made in USA) with proper connector from states so did not have to use crimping tools. including postage was around $100. In comparison with what Toyota suggested - 500 + GST or Repco (non genuine, use crimping tools) = 200 I think i've got a bargain.
FYI - Oxygen sensor Trouble code is 21.

Spark plugs - check the gap and overall condition.