89 4.9l I6 w/130,000 on it. Ran some Marvel Mystery oil in it for a couple hundred miles, changed the oil. Replaced with 5w-30 and a purolater gold filter. Cranked it up and now it has a knocking sound and I can hear the lifters knocking a little.
It's never done this before. Also oil pressure used to run at the "o and r" in NORMAL
but now it is halfway between the white line and the N in Normal. I'm thinking that I might have to replace the oil pump? Your thoughts / experience with this would be much appreciated.

 

It would be good to know the actual oil pressure to know if in fact it is a pump problem. If not the pump, 10w/30 w/ 20% lucas oil stabilizer. Wondering if sufficient marvel residual remained to effect viscosity of the replacement oil?

200 miles? Did you forget it was in there? You might want to check your compression. There is also a protective glaze on the cylinder walls that can be adversely effected, causing the rings to loose seal! These symptoms can occur w/ the use of "Seafoam" if used incorrectly also. Not from a single use, but if used redundantly over a short amount of time.

Then again, how much Marvel did you add to your oil? 20% should not have presented a problem in 200 miles. I used to add it to my gas as a upper cylinder lube. Now I prefer to use lucas upper cylinder lube & cleaner.

 

About a half pint of MM. And I have used Seafoam in my tank. I have used it twice in about 500 miles or so. I don't recall reading that it should not be used in short successions (Seafoam). I cannot tell what the numbers are on the OP due to I only have a guage with no numbers, only the word "NORMAL" on my OP gauge. Sounds like I used too much according to what you are telling me?

 

I'm referring to adding a can to your oil (Seafoam). As a fuel additive, not as much an issue. You're ok in that respect. & 1/2 pint of Marvel in your oil should not have pooched things either (20% would be a quart). Starting to look more like the oil pump. I hate coincidences!

Not saying you can't screw one up, but 4.9's are one tough engine!

 

The MM might have dislodges some crud that is now blocking the intake screen of the oil pump.

 

OK I'm not totally familiar with these engines, Isthe oil pump external or isit in the pan. Thinking about dropping the oil and rplacing again. This bothers me bad because at 130,000 this motor has never leaked or burnt one drop of oil.

 

Yep. Like I said "Coincidence"?
In the pan. It would bother me too. 130K ain't **** for a well maintained engine. especially the 4.9!

 

the bigger issue could be that you used 5w-30. The engine calls for 10w-30 a slightly heavier (thicker) weight oil. I'd drop the oil replace it with 10w-30 and substitute one quart with Lucas Oil Stabilizer.

I'd bet your noises go away.....

 

That was suggested.

Hackmiester! Rumors of your demise have been greatly exaggerated I see!

 

x3....5w-30 is the problem......especially on a 130k miles motor.....the recommended viscosity pretty much goes out the window after 80k if not before then.