I have to ask are you on retard pills? Why would you be running mineral spirits in a fresh engine? Your posts and what you are doing make zero sense to me.

Could the thick stuff be moly from the assembly lube. As Mineral spirit is a highly refined product of oil in the first place, I dont think the thick crud is insoluable in oil. If you are concerned about running the motor with the foreign stuff in the pan, why not just run the oil pump? Pull the drive gear and chuck the pump with a hex shaft. The lifters are probably still good, just remove the circlip, disassemble and soak in...mineral spirits !

Mineral spirits wont do what you are describing if that is what you had in the crankcase. It mixes perfectly with oil and just thins it out; does not form a sludge of any kind. Sounds to me like you are getting coolant in the oil. That will make a gooey sludge that sinks to the bottom of your drainpan.
-dulcich

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or else you have coolant/ water in the oil.

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Have you checked your coolant level?

I agree with above you need to be checking for a coolant leak.