I think I started my engine on the way to ruination and I do feel
stupid.

Vehicle is a simple Voyager, 1994, with 203,000 miles.
It was not smoking although it has the infamous 3.0 liter Mitsubishi
engine.
Very tight engine considering the miles and its heritage.

I was changing the oil every 3000 miles.

I let the oil go for 4,500 miles and 9 months instead of the usual 3
months.

But what is worse, I added 2 quarts in 9 months of plain SAE 30 weight
oil.

It had 5W-30 Conoco-Phillips semi-synthetic blend which was doing well.
Before I would add 10W-30 regular oil to the blend. My poor logic was
that SAE 30 is a single weight and less interfering with the 5W-30 and
since the temps were above 32 F, should not matter.

Now idling, I hear something. It idles rough. It is smoking at idling.
Don't think this is normal. But under way, it does not smoke and is
quite smooth. The engine sounds fine if moving along and not idling.
That is encouraging. Don't hear any strange noises when moving.

Also, did not replace the oxygen sensor for many months. Did quite a
bit of non-highway driving. Was told this cannot harm the engine but
only the catalytic converter. Not sure about all of that. The mileage
is quite good even with these problems.

I hear the valves tapping upon starting. Usually goes away in 2 minutes
now. Used to go away in about 15 seconds.

I am not sure if the noise I hear is the alternator going, motor
mounts, or other things fixable. I'm concerned about valve damage,
engine knocking, and even piston slap. If I did do damage to the
engine, which seems unlikely, but something occurred and it may or may
not be a coincidence.

What do you all think? Tell me even if it's not pleasant. I think I
pushed the poor engine to accelerate its aging. It's too old to take
even the slightest abuse.