Thanks to all of you who responded (both on or off list) about my new
engine/rebuild questions.   After fixing a bizarre problem (loose
throttle body -- see other email), I'm back to square one with
regards to my evaluation.

The situation:  85 westy automatic, 150k miles, automatic, CA emissions.

- When warm, low oil pressure light comes on under 1100rpm.  I'm
running 20w50 with a good filter and it has already had the 0.3bar
switch mod.

- When cold or warm, has rough idle, with lots of little backfires
(pop).  If  you give it the slightest bit of throttle, the idle
smoothes out.  Mechanic thinks the lifters are not lifting due to low
oil pressure at idle, causing the rough idle.

- Runs fine on the freeway -- I can cruise at 70, get about 17mpg,
and it passed emissions last year.

- Mechanic said "quite a bit of" metal in the oil at last oil change.

The question:  Do I need a new engine, and if so, how soon?

Since I'm not terribly impressed with my mechanic's diagnostic skills
at this point, are there any other things that could be causing a
rough idle with backfires at idle?