I have the 1MZFE motor in my 2003 Sienna.
This is outside the "gelling" program years.
While the rear valve train should be more dirty than the front as that is where the air exits through the PCV valve......with the front valve train should be cleaner, as this is where the clean air comes in through the breather......
you can STILL get a very good idea of what is going on by removing the front valve cover.
Replacing the valve cover gasket would be a good idea.....and they are cheap.
Before assuming that there is a sludge issue, I would take a look-see under the front valve cover.....you might find that you are in better shape than what you are thinking.
Now, as long as your vehicle has had a good OCI up to this point, the main thing to look for is to verify that you have flow through the PCV valve.
A clogged PCV valve will lead to "gelling" or sludge in short order.
If the baffel system in the rear valve cover should become clogged, then the same thing would happen.
HOWEVER, if the front valve train is fine.....then most certainly, the rear is OK also.
Here is a link to the first in a series of pictures on the front valve cover removal on my 2003 1MZFE.
http://rides.webshots.com/photo/2609915850011220610XGbmsa
My 2003 has a different PCV valve as compared to earlier years, however the location will be the same.
If you wish to remove the rear valve cover, then you will need to remove the cowl between the windshield and the hood and the upper intake manifold.
I have pictures in the same album that show this.
The cowl may be different on a different vehicle, but the upper inatake manifold will likely be very similar.....with slight differences between years.
For the upper intake manifold....I left the throttle body attached and just moved the assembly aside enough to get to the rear valve cover.
The upper intake manifold has supports at the rear.....with different size bolts.....that you cannot see.
I show these in my pictures.....as well as the coolant lines to the throttle body.
However, this is not a good job for one with a bad back as you are leaning WAY over and reaching a good distance.
So TAKE YOUR TIME and take rest breaks.....to keep a good back from going bad.
Contrary to what one might think in the pictures, the valve train (front and back) were in good shape.
The darkness was a thin film that wiped right off......I had a double dose of AutoRX in there (which is black and supposed to "cling" to metal during the "cleaning phase").
Also some dark areas are rough casting spots that are wet with oil.
My particular motor saw 5K OCI with valvoline synthetic....and has about 87K miles on it at the time of the pictures.
I have Redline 5w30 in there now, along with the oversized filter (WIX 51516) which is identical to the Napa Gold 1516 you list.
I expect that the Redline will clean things up more over time.
I am certainly not concerned about any sludge issue with my motor.
I also have pictures that show the transmission fluid and filter change, using Redline D4, as well as a auxillary transmission cooler and magnefine filter installation.
Unless I'm very mistaken, the '98-'01 RX-330 shares the complete same drive train as the '98-'01 Sienna. And indeed I've read sludge stories for the RX-330 during that time frame.
