The parafin based oils will, and do, sludge up an engine. Some brands are worse than others though. Back home we had a parafin based brand called "Amalie", which was touted as being 'better than it had to be'. I never used it, and after buying a Pontiac with the 400 HO, that had spent it entire life on Amalie, I vowed I never would. When I pulled the valve covers to replace leaking gaskets, I found that the top of the heads were completely encased in a waxy paste. The push rods and valve springs were completely surrounded with this stuff.
It took a lot of digging and scraping to remove most of this crud. Afterwards I drained the oil and filled the crankcase with diesel and ran the engine to clean the innerds. Took a couple diesel rinsings and cheap oil flushes, but I got her cleaned out. I never did have to crack open the crankcase so I don't know how bad the internal passages were clogged. But based on the tops of the heads, I have to imagine the were pretty bad.
73 280 SEL 4.5