Toyota Repair: 3vze Knock, engine temperatures, stethescope

Question
Ted,

I took my 1988 4runner in for diagnosis of the knock we had discussed a couple of weeks ago.  The knock is not very loud and only occurs at warm engine temperatures for a brief instance at low rpm under light load.  

Here is what my mechanic did and the results he came up with:

1)Warmed engine to 192 degrees.  With stethescope he listend all over the engine block.  He pinpointed the noise to be the loudest 6 inches back from the front on the driver's side of the block, oil pump location

2) He drained and collected the oil.  Then using a camera/scope inspected the crank and rod journals through the drain plug.  He found no evidence of blueing or excessive heat to indicate a bearing failure.

3) He tested oil for possible metals to indicate bearing wear and none was found (oil had been changed 200 miles before this).  He thought even after 200 miles copper would have showed up if bearing was failing.

4) He removed spark plugs and examined the cylinder walls for scoring with camera/scope for wrist pin damage.  None was found.  In fact after 205,000 miles the factory crosshatch was prevelant.

5) He tested oil pressure.  I had pressure of 7.4 psi at idle and 36 psi at 3000 rpm.  These all fall within spec.

6) He manually rotated the crank and listened with plugs out.  When rotating the engine back and forth a noise is heard at oil pump area.

His diagnosis is excessive clearance in oil pump.

He said he wouldn't worry about it as long as I have good pressure, but to keep an I on the gauge.  He thought that complete failure of the pump would be unlikely.

Is this diagnosis credible in your opinion?  Even an oil pump replacement is $1000! I guess it is some serious labor to get the oil pan off a 3.0 3vze. I would hate to replace the oil pump and still have a problem.

But my mechanic said not to be concerned with it. He did recommend switching to 20w-50 oil in the summer and 10w-40 in the winter.  Currently I run 10w-30 all year.

Thanks again Ted  

Answer
This used to be a very common issue with the 3VZN engine, the noise is coming from the front main bearing, the bearing has a little too much clearance which causes the low pitched thumping type noise, my own 4-Runner did the same thing and I actually drove it for a long time but the noise became annoying so I finally replaced the bearings.
The main bearings have numbers that indicate the clearance between the bearing and the crankshaft journal, they are numbered from #1-5, the solution is to replace all the rod and main bearings and reduce the clearance of the front bearing by installing a #5 bearing,all of the other bearing numbers should stay the same, using a different oil viscosity won't be of much help.
To verify the condition of the bearings as I have described, remove all of the drive belts and see if the noise goes away. Doing so will remove pressure from the front pulley and lessen the noise, a lot of times there were complaints by customers of this noise after a timing belt was replaced, the new timing belt and the tighter drive belt adjustment would cause this noise occur.
I have never experienced an oil pump causing a knocking noise.