Coincidence? or not?

1996 Chevy 1500 Silverado 350 Vortec sbc. 120K miles stock. Ran perfect. Noticed a tapping sound coming from the left bank. Sounded like a lifter. Pulled the cover and all looked tight. Adjusted the valves on that bank and the tapping lessened but was still there. Later that day I noticed the oil pressure down around 40psi (normally runs 60psi). I changed the oil and filter with 20w50 (from 10w30). The tapping noise was a little less and pressure was up to about 48psi. Two days later, steam blows from under the hood and the crank is filled milky white. I assume it blew a head gasket. Pulled the engine out and decided to get a long block from Autozone. When I pulled the distributor cap I noticed the rotor only had one screw holding it and was spinning off center wearing on the dist. housing.
Could the timing cause a head gasket problem?
How about the valve adjustment? I went a 1/2 turn tighter on all the valves.

-Kevin

sounds coincidental that the screw was loose at the same time the haed gasket went, as for the tapping probably a weak lifter or a failing cam lobe, also i think 20 w 50 on a stock motor is not the best choice, more of a racing application. and do you mean 1/2 turn farther past the original adjustment? if so thats probably i little tight and the valves will be at the point where they might not seat properly on the down side of the cam, and it will wear quicker than ever that way.