I'd put in about a cup of ATF to start if you're going to go that route. Shitload of detergents in there so it should clean it out nicely.

I use Shell Rotella T6 5w-40 which has a decent amount of detergents since it was designed for diesels, so I'm fairly certain my engine is kept clean each time i'm driving it LOL

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2018 Ram 2500 6.7L Cummins 68RFE
19k miles -Bright White/Black - Big Horn Sport - Crew Cab Short Bed
2013 X5 35D (CEO's) - Born on 5/17/2013 -
82k miles - Alpine White/Cinnamon Brown/Premium Pkg, Sport Activity/Premium Pkg and Sound/20" Style 214/Running Boards