I went ahead and dropped the pan on my 62 C10. I was very glad I did ! The sludge was probably close to 3/4 inch deep.
While the pan was off I got a good look into the crankcase. I was surprised. Other that the sludge in the pan , it was very clean. Almost new looking. This forum helped me find out the engine had been replaced at some point with a rebuilt one 👍

1962 C10 with a 235 6cyl -- all of the drive train seems to be original.
Some of this story is in the Side Lot
Some people like a new truck. I liked the old ones.