i've done a search and came up with maybe leaking spark plug gaskets or seals . i checked my manuals (the haynes and the 100 dollar toyota one) and there is no say about them. i even took the spark plug pipes out to see if there was anything under them. nothing.
are the seals right under the 4 valve cover nuts??? i didn't think those were the ones the were posted about, cause i don't see how oil could leak in to the spark plug pipe.
I had that problem with mine. 2VZFE seals go between the Rocker cover and head. The chamber in which the spark plug goes down is not just all head, it's part rocker cover too. So, take your rocker cover off and you should see them.
my good friend. this engine is an ohc engine correct? (over head cam) take your valve cover off and take out your camshaft(s) look under your camshafts or where they were. you should see flat round pieces of metal. they look like disks. your valve seals are under them. if you want to replace them you will need to remove the head and take it to a mechanic or preferably a machine shop. this process requires a special tool called a valve sping compressor. the seals are right under the disks pressed against the head. hope this info helped.