Is 5w-30 oil and no thermostat a big no no in winter?

[AntiqueAtHeart]

Ive been driving my '66 long bed fleetside for about a year now. Ran it with 10w-30 and no thermostat in the summer and the temp never topped 200*. Now that its ~40* to 50* outside just about everyday, I switched to Mobil Super 5w-30 conventional oil and still have no thermostat due to lack of time to put one in. I've been driving everyday for about a month this way with an average of about 10 miles a day. The oil currently has 1700 miles on it and the engine is a 350 chevy with a Comp Cam 292 and stock valvetrain except springs and pushrods. I've put almost 4000 miles on it since rebuild.

It has now started to rattle badly like loose valvetrain and is missing on one cylinder. I'm pointing to the valvetrain......bad lifter.....

Did I kill my engine with thin conventional oil and no thermostat in winter?

I sure hope not. I'm going to try to pull it apart tonight to see whats wrong........I need coffee.....

-Cam