OT mechanics - Best engine cleaning additive (oil)

Booty sweat

Marvel Mystery oil 1/5 ratio (20%), couple hundred miles.

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Are you currently running synthetic?

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Yes, but not sure what was run in it for the first four years (60k miles)

Drain out a quart of oil and replace it with Diesel Fuel. Idle engine for 10 minutes and drain case, fill with new oil.

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I see all kinds of additives, but what is the best?

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Most decent mechanics will tell you not to do engine flushes. Most of them are snake oil and you are just paying for some kerosene and a few solvent additives. MMO and Seafoam have limited solvent capacity in the amounts you can safely run in the engine, they aren't going to do much when there's a lot of hard deposits and varnish involved. The best way to go, assuming you don't have an engine that has stuck rings, is to simply start using a quality synthetic oil/filter and change the oil on shorter intervals. Most oils have increased detergent additives now, synthetic oils especially and they can clean out an engine gradually.

Don't put any of that shite in your engine. Just run a good oil and increase frequency of filter changes. If you are really neurotic about it, start doing oil analysis and install an additional bypass filter.

If your concern is carbon build up on the back of the valves….how would an oil additive help? Wouldn’t you need to focus on the intake? Then a catch can to prevent it from recurring?

Honestly I wouldn’t worry too much about it unless you have a Volvo, VW, Honda, or Audi with a turbo + DI. For whatever reason, their turbo DI engines have either a overly simplistic PVC system or they aren’t using adequate baffling to catch the blow by, enabling all that garbage to get baked onto the back of the intake valves.

GM, Ford, and Toyota do pretty well with it with their newer DI engines especially in their trucks. A lot of people do not like the smaller displacement turbos, but the GM Turbomax and the Ford Ecoboost motors have actually been pretty solid. In some cases they are adding port fuel injections to their DI engines too which is the nuclear option against carbon on the back of the intake valves - at least until the EPA objects to that.

If you have a Ford 5.4 or one of the other engines prone to oil sludge….change it often with high quality oil. That’s really good advice for any modern engine. Lower weight oil is used these days and they are deploying these variable pressure oil pumps (to reduce load on engine and increase MPG) that can potentially starve engines from oil in some circumstances. That’s what’s really hurting the lifters in the Ford 7.3L gas engine and some of the Chevy V8s from 3-4 years ago. Only way to combat those issues is to change the oil often and use the top quality synthetic. And listen carefully for a ticks so you can address them before metal parts start flowing through the motor.

If you only have 4 or 5 years on the motor I don't think you should have much build up issue.
A little additive to your gas every so often would not hurt.
The turbo motors do build up some due to the excess blow back. If that is the concern look up a catch can for your vehicle. They are easy to install in most vehicles.

Stop buying cheap gas and oil, change regularly, do regular maintenance, don't worry about buildup.

If you're REALLY paranoid, get an oil catch can at the PCV, it'll capture fuel and oil before it gets back to the combustion chamber. Thank EPA for that engineering stupidity.

If you can find it, use BG EPR. Pretty much only sold to repair shops as far as I know, but it works on crankcase/valvetrain/vvt carbon deposits. Idle for 10-15 minutes after adding it, then change oil and filter.