I've used MMO in engines I suspected of having contaminant problems due to age, long term neglect by previous owners, etc - in general I feel it's a fairly safe and effective solvent cleaner for the oil. Difficult to guess at how its viscosity would affect MDS, it's a very light oil so a substituted quart might make your 5w20 behave more like an imaginary "5w17" .
As for when to add it, you can just substitute a quart of your regular oil at an oil change and then run it the whole duration of that interval. Your truck really shouldn't be consuming oil with only 135k miles on it so if you filled it to the top of the hash lines on the dipstick when you did a previous change you probably don't have room for a whole extra quart to add mid-interval.
But I also look at it like taking Tylenol when you already feel good - not really going to help anything is it? If you don't suspect a problem that can be addressed by the MMO then you're just sort of doing a little dance for nothing. If your truck's been taken decent care of there should be no work for the MMO to do at just 135k miles.
I bought a 2007 Kia Sorento from the salvage pool on August 7th, 2019. This wasn't a flood car or wrecked; it was a neglected high mileage car whos owner should be banished to a bicycle
.
She had let the oil changes go over 10-12,000 miles many times in it's 203,000 mile life of hardship.
The ABS hydraulic unit had 2 seized valves which resulted in braking on only the left front and right rear wheels! Talk about a challenge to stop it. It was horribly filthy from her heavy smoking and hauling 3 large breed dogs in it.
The Cabin air filter was not only stopped up, it had 1/2 inch of mold growing on it.
I pulled the one valve cover off that could be accessed without removing the intake.
It was literally solid sludge there and in the oil filter housing! One result was a P0016 Cam/Crank correlation code for bank 1 which never went away, but did play hide and seek.
I had it professionally detailed and fixed the ABS unit with a salvage unit. I then began trying EVERY product made to dissolve the sludge.... NOTHING would actually dissolve it, but break it down to coffee ground sized chunks.
Eventually, the oil pump suction screen became overwhelmed.
After 2 attempts at cleaning the screen, I parked the car and found a salvage engine in Midland, Texas with 91,000 miles on it. I paid for it and had it ground shipped to my shop. After letting it sit for a tad over 6 months and gathering parts I needed to replace while I was performing an engine transplant, I spent a week and a half replacing the engine.
The donor engine had zero sludge and was "certified" by the salvage folks I used. They provided documented proof that their claims were true. I took the VIN number off the block and Car Faxed it... and found that the vehicle from which it cam had been properly maintained.
My takeaway from this among the other many times I've attempted to "desludge" an engine is that it isn't possible. An engine that far gone is totalled.
We did get another 30,000 miles on the sludged engine in 4+ years.
