The oil filter is a real bear. It is up under the engine at an angle. So when the center bolt is loosened it will drip oil down your arm and onto parts of suspension. The cannister must be tilted even more to get it out from between members. The filter has two parts. A reusable nylon full flow filter and a replaceable element which I think is the bypass portion. The nylon part should be washed in solvent. The manual shows a special tool to hold and seal the opening in the filter while it is washed in solvent. Be VERY careful that the cannister is correctly seated as you retighten the center bolt, or it will pump large quantities of oil on the ground. Been there, done that.
I would keep spinning the cannister as I tightened the bolt to be sure it was correctly seated. Must do it by feel, can't really see it well enough to tell.
The topside filter on the W123 was a huge improvement.