Many years ago when I took high school shop class, there was a late-70's Buick in the shop with engine failure. The cause of the engine failure was the rocker valley underneath the intake was packed completely full with sludge. So much that the valves could no longer travel up/down and they bent.
Agree with changing oil is one issue, but also using different viscosity's of oil intermixed can increase the likelihood of sludge build up. From your picture, I would be sure there is more sludge elsewhere.
