2000 Sienna PCV / Oil Leak / Check Engine Light

It's been quite a while since I last posted.  My 2000 Sienna, with
just over 153,000 miles so far, has been very reliable.  I do most of
the routine maintenance and minor repairs myself..

My oil change interval always has been 3000-4000 miles (usually in the
low 3000's).  For the first 15,000 miles, I had used conventional
Mobil 5W30.  Since then, I've been using Mobil 1 -- most recently the
"high mileage" version that is formulated for better seal swelling
properties.  I also change my pcv valve (using a Toyota OEM part)
every 15,000 miles.

When I changed the oil 3000 miles ago, I couldn't get the usual Mobil
1, so ended up buying WalMart's house brand, fully synthetic 5W30.
The purpose of this post is not to malign WalMart, because I've read
that their full synthetic is a very high quality oil.  However,
unfortunately, after a few weeks of using this oil, I started noticing
that oil was oozing out around the valve cover gasket, and dripping
onto the exhaust manifold (and burning) and/or onto my garage floor.
I attribute this to the fact that this oil probably doesn't swell the
engine seals as effectively as the high-mileage Mobil 1.

After the WalMart synthetic had been in the engine for about 2000
miles, the "check engine light" came on.  I noted that the oil still
looked nice and clean, and I'd only lost less than 1/2 quart due to
seepage during those 2000 miles (and I don't think I'd used or burned
any oil).  Also, there has never been any smoke coming out of the
tailpipe or any other sign of engine sludge.  For some reason, I
decided to investigate the condition of the pcv valve (which also had
only 2000 miles on it).  I found that it had a coating of oil on the
intake portion of the valve (i.e., the part that is inserted into the
rubber grommet in the engine manifold) and, most importantly, it
seemed to be clogged.  I shook it, but there was no rattle.

So I replaced it with a brand new pcv valve and, lo and behold, after
just a few cycles of driving the van, the check engine light went back
off and stayed off for about 1000 miles.

Just a few days ago, after about 3100 miles with the WalMart oil, I
did another oil change (back to the Mobil 1 high mileage oil).  The
WalMart oil still looked pretty good.  There had been no large loss of
oil, and I saw no sign of any sludge or solids in the drained oil.
But the next day, after driving around a little with the Mobil 1 in
the engine, the check engine light came back on for the first time in
about 1000 miles.  So I checked the new pcv valve (in use only about
1000 miles), and it once again had a coating of oil on its intake
portion, and it also seemed clogged (i.e., would not rattle).  So I
cleaned it out with a carb cleaner spray, blew it out with a
compressor, and it seemed like new again.  I reinstalled it and then,
today (after just a few cycles of driving with the clean pcv valve),
the check engine light went off again, and has stayed off.

I don't have an OBD reader, so I don't know the code for the CEL event
that just occurred.  I've read that a clogged pcv valve would NOT
trigger a check engine light.  However, this is what seems to have
happened these last two times.

When the van reaches about 180,000 miles, I plan to bring it to my
local garage to replace the timing belt and to have the valves
adjusted.  At that time, I will ask them to look closely to see
whether there are any blockages in the engine, and hopefully they'll
be able to stop any remaining new leaks when they install new gaskets
and seals.  Between now and then, I intend to clean or replace the pcv
valve at every oil change.

Does anyone have any ideas about what is going on, or any suggestions
about this?  Thanks for your help.