It sounds like you might want to consider both a top-down cleaning to address the mediocre compression and a bottom-up cleaning to address a neglected crankcase. Below is what we would recommend for both matters.

TOP-DOWN
For a “top-down cleaning,” we mean performing a cylinder soak to clean the piston crowns and compression rings and landings. To do this, remove the spark plugs and pour Berryman B-12 Chemtool Carburetor, Choke & Throttle Body Cleaner (Part #0116) into each cold cylinder in quantities sufficient to cover the entire pistons and then thread the spark plugs back in finger-tight.

In an engine with an inline configuration like typical 4-cylinders and straight 6s, this may only be an ounce or so of product per cylinder. In a V-configured engine with cylinders at 45°, it may take nearly an entire can of 0116 per cylinder, in which case you might consider jacking up one side of the vehicle and soaking just that high bank and then repeating the process on the other side.

A typical soak time is a couple hours, but if leak-down is minimal, you can soak overnight, too. (A little 0116 in the oil is not problematic as the solvents will boil off the first time the engine reaches operating temperature.) For best results, rock the vehicle occasionally to gently agitate the cleaner. Remove the spark plugs, suction out the combustion chamber, and reinstall the plugs and plug wires.

Once you’re done with the cylinder soak, we recommend using an in-tank fuel additive for PM. In order to keep the fuel injectors, intake valves, and combustion chambers clean, try Berryman B-60 High Mileage Fuel System Rejuvenator (Part #7516) every 5-10,000 miles. One can treats 15-21 gallons of gas and will thoroughly clean these components in PFI and TBI engines. (For GDI intake valve cleaning, look into applying Berryman [2611] about every 10k miles.)

BOTTOM-UP
For a “bottom-up cleaning,” we’re talking about using an engine flush immediately prior to performing an oil and filter change. For this task, we recommend using Berryman Oil Change Flush https://www.berrymanproducts.com/products/maintenance/berryman-oil-change-flush/ which is not like the old-school engine flushes with harsh chemicals that can dislodge large chunks of sludge, plug oil screens, and starve the engine of oil. Instead, this flush is a gentle cleaner that attacks crankcase buildup little by little. As such, it may take 2 or 3 oil change intervals to completely clean a neglected engine.

To get a good feel of the degree of cleaning the product is doing each time, take a couple different types of paper—newsprint, computer paper, etc.—and place 1 drop from the dipstick on each type. Then run the flush for 10-15 minutes as directed, and apply a fresh drop of oil to each type of paper and note the color change. When you no longer see much difference in color, you know the crankcase is clean.

Once you’re on top of crankcase cleanliness, you generally shouldn’t need to perform an engine flush but every 2 to 3 oil and filter changes. That said, you can safely run the product with every change for maximum cleanliness, if so desired.