A Forum Search of "Sludge Trap" will bring up all you need to Know. Obstruction of this internal oilway connecting the two big end journals will result in seizure or catastrophic failure of the drive side rod. No if's or buts, if the crank is ever removed it is well worth removing the plugs and extracting the internal tube and its deadly contents found on big journal cranks with radial flywheel bolts.
Small journal cranks have a simple oil cross drilling, cleaning this is a little less complicated.
Here's something to demonstrate what's what, pictured is a small journal crank from my original plunger motor........ It needed a little heat to get those pesky plugs out.
Swarfy.