I have a 2003 Sienna.
I have NOT relieved the fuel pressure, but did not need to do that in order to remove the front and rear valve cover.
I would order some Felpro valve cover gaskets.
Also, while you are doing the work of removing the rear valve cover, this would be an ideal time to change the spark plugs, if they have not been changed.
I would use ONLY the DENSO or NKG plug of the number specified as OEM for your year.
This number should be listed on the under hood sticker and in your owner's manual.
They used different plug numbers for different years within the 1998-2003 model years.
My 2003 uses a metal PCV valve that screws into the rear valve cover.
Earlier years use the traditional press in version.
I would install a NEW PCV valve.
Once you clear up the sludge, it will be CRITICAL to check the PCV valve to make sure that deposits do not build up on it to the point of restricting or blocking airflow through it.....or you WILL develop sludge.
Maybe every 6 months to a year between inspections.
As mentioned, if you have sludge in the valve covers, the TOYOTA solution is to REPLACE the valve covers.
The reason for this is that there is a baffel that is spot welded in place, which covers much of the "top" of the valve cover (inside).
Once sludge (which is hard) builds up between the top of the baffel and the bottom surface of the top of the valve cover, it WILL block flow of air into the front valve cover through the breather and out of the rear valve cover through the PCV valve.
Trying to disolve this buildup (above the baffel) with solvents has not been successful, which is why replacment of the valve covers is the way to go if they are sludged up.
If you do not have this airflow, you WILL develop sludge.
THIS IS CRITICAL.
The front valve cover is not hard to remove.
The rear valve cover is a bit of work.....you need to remove the intake manifold.
I did not separate the throttle body from the intake manifold or disconnect any fuel lines.
I was able to move the whole assembly enough to access the rear valve cover.
There are some bolts that you have to remove that hold the upper intake manifold to a bracket on the back side of the motor.
I show the location of these, and the bolt sizes in my pictures.
Again, note that I have a 2003, but the bolt locations and sizes should be pretty much the same for other 1st gen. Some of the vaccum lines, and grounds, etc will be different and the 2003 has a dual throttle body.....so the throttle body assembly is bigger.
Manuvering the rear valve cover around that big bunch of wires back there is a job......be SUPER careful to avoid pulling the rear valve cover gasket out of place (particularly the back side) or you will have a nice big oil leak back there.
The upper intake manifold on my 2003 has a couple of 8mm hex bolts, so you will need a 8mm hex bit....I bought a set at Autozone (your tranny drain plug takes a 10mm hex bit).
NOTE that there are coolant lines to and from the throttle body.
Your 1998 does not have the VVT-i on the intake cam, so you will not have the VVT cylinder on that cam, but otherwise yours should be very similar.
Use CARE to avoid knocking chunks of sludge down into the motor from the area under the valve covers......as this could block oil flow.
My pictures show the slightly longer oil filter that I use.
This would be the SAME size as for your 1998.
A good toyota dealership is a good source of quality parts.
Genuine TOYOTA parts are often very competitive in pricing with aftermarket.
A link to my pictures.
http://rides.webshots.com/album/576572371kcnZsT
I had a local TOYOTA dealership replace the timing belt and water pump as it is a more involved job than I am comfortable with.
They were cheaper than many independant shops for the same job at around $500.
The timing belt change interval is 90K miles.
The spark plugs are rated for 100K to 120K miles (but getting back to remove the rear valve cover, you have done 90%+ of the work involved in changing the spark plugs).
Most likely the actual cause of the oil light coming on was the clogged pickup for the oil pump that you found.
If you knock a bunch of chunks loose up top.....they could well clog the pickup screen again.
One product that I have used as a oil additive for cleaning is AutoRX, as it is a slower, more gentle cleaner (disolves rather than chunks) than a "motor flush".
If you use AutoRX (available online), follow the instructions on their website.