I would discourage any attempt to flush out your oil system. Manually cleaning the sludge or debris out of the pan is fine and that is where most sludge and debris ends up. Running gas through it won't do much, unless you give it a long soak time. Usually some manual effort is required to get it out. However, any sludge, debris or varnish that you loosen in the oil galleries or tributaries runs a high risk of getting lodged in smaller downstream clearances, such as lifters, pushrods, prop governors, etc. and creating problems that did otherwise exist.
This is very common in automatic transmissions (much smaller passages). Owner neglects trans for 90,000 miles then tries a flush with a lot of detergent. This breaks everything up and is free to float around and clog circuits. Next step is a complete rebuilt to get it to shift properly again.
The general wisdom is that if the build up is not causing problems, leave it alone. Trying to remove it is high risk, low reward.
Do you have reason to believe there are builds up in your system that are creating problematic restrictions? If you are infinately curious, you can remove the two clean out plugs in the main gallery and inspect it. However, resist the urge to run a brush down through it. Most of what comes off will go down the capillaries.
Additives, such as MMO, will typically only thin out sludge and do so very slowly. They also don't typically break off hard solids. For these reasons, they generally do no harm and will help with heavy sludge buildups. Any sludge dissolved will typically end up again as sludge in the bottom of the oil pan where it does no harm.
The best way to prevent sludge is frequent oil changes. Sludge generally forms when the oil's capacity to keep debris in suspension is exceeded(i.e. becomes fully saturated) and it settles out via gravity, though it can stick to cavities the oil is flowing through. This exclude the Gelatenacious gunk that forms from excess moisture and limited full temp runs.
Larry