DC oil filter - optional?

If you are not going to use a filter then you should only use non-detergent oil. HD oil keep stuff in suspension, witht idea the filter will get it out. With no filter the bad stuff just keeps getting pumped through bearings and the like. Non-Detergent oil lets the bad stuff settle out (hopefully to some non critical location like the bottom of your oil pan)

These engines ran non detergent oil for most of their early life, and possibly much longer. The engines were designed so that the oil could drop out dirt and sludge in the oil pan. The filters became optional later as Tom said, but they are a partial flow filter, so the pan still caught the big stuff. To clean the engine, you need to pull the pan. Alternative is to just pull the inspection covers and scrape, wipe, or shovel as appropriate. I am not making a bad comment about your engine - I have taken apart several Ls and LAs, and they had some scary stuff in the pan. You may also find that the screen assembly on the oil pump intake has come apart. If this is the case, I can suggest some pointers on its repair.