I've run a lot of MMO in my old Chevy K1500. A lot of MMO in the 50k miles I've driven her.
She was sludged up pretty bad when I got it. The PCV port was plugged in the throttle body when I got it, and I have no idea how long it had been that way.
There was a lot of crusty black crud under the valve covers. I know that stuff was also in the crankcase as well.
Since my truck was old, and a small block Chevy, I had no problem running 10w40 or even 20w50 in it, along with a quart of MMO. I've also run 5w30 with 20% MMO on board as well.
If you have a verified gunky engine, so out and buy several cheap filters and do several 1,500-2,000 mile OCI's with 20% MMO.
Something else I did to help get rid of the sludge was to drain the oil and fill the crankcase with a 50/50 mix of kerosene and paint thinner. I added 2 quarts over the oil capacity, and let it sit for 2 days. I removed the wire connecting the coil and distributor and cranked the engine over for 10 seconds about a dozen times each day.
The kerosene/paint thinner mixture came out black as coal.