Engine flush

I saw two coworkers do this in the '80s to two large cars they bought when gas prices were high and big cars were cheap. They added one quart of transmission fluid, then drained the oil a few days later. They had good luck with that procedure on other engines. I never had the guts to try it.

I did buy a used car that needed to have the valve covers replaced. Found 1/4" of sludge inside them. I added a second, bypass oil filter that used a roll of toilet paper. 60,000 miles later had the valve covers off again and they were spotless inside. I still use that toilet paper filter on a 1980 model car I have now.

You might also consider that engine oil is constantly going through improvements to meet the engine manufacturer's needs. Those improvements mostly involve the additives, including detergents, dispersants, seal conditioners, anti-foaming agents, and corrosion inhibitors. If you aren't having any other oil-related problems, consider changing the oil and filter every 1,000 miles or so two or three times. The detergents will dissolve some of the sludge, then the dispersants will carry it to the filter. Once the filter cools down, that sludge can solidify in it and block it. There's always a bypass valve in the filter or in the housing it screws onto. When the filter is plugged, oil is going to bypass it to get to the engine, but it won't be filtered. That's why the filter must be replaced during each oil change. With each oil change, the new, clean oil will dissolve more sludge until it's all gone.

Use this guide for more information:

https://www.2carpros.com/articles/how-to-change-engine-oil-and-filter

This video is specific to your model:

https://youtu.be/lAH8YkdYnks

No experience with Marvel Mystery Oil. I do use it for other things.

Going way back to the late '70s, I've heard many stories about synthetic oil being "cohesive to itself", meaning if you have a leak, the oil will continue to pull itself out making the leak appear to be worse. A former student had a late '80s Ford truck with no oil leak. He switched to synthetic oil, then within a week it developed a huge leak leaving a puddle everywhere he parked. After two months, he switched back to conventional oil. The leak became much less, but it never totally cleared up.

My suspicion is the two types of oils used non-compatible additives. Evidence of that can be seen when someone changes brands of oil even though both are conventional. It's common to see leaks develop, regardless of which brand you've switched to. An old seal conditioner additive might be dissolved by a detergent in the new oil, for example.

I use only Valvoline in my 1980 Volare, only because it was the least expensive brand back then. Mobil One synthetic goes in my 2014 Ram since I got it at 4200 miles. My old rusty trusty '88 Grand Caravan gets whatever is the cheapest farm and home store brand at the time. Never had a single oil leak in 440,000 miles. My cousin believes in Pennzoil. Many people do not.

Do you have a history of how or why this sludge developed? If it was from neglect by a previous owner, I would just do regular oil changes, perhaps a little more frequently than normal for now. I don't see an advantage to switching to synthetic oil.