There are some interesting threads on the Corvetteforum regarding sludge in the radiator. It seems that when certain types of antifreeze are mixed together they can form sludge. One of the the posts also goes on to describe a case where the antifreeze was mixing with the engine oil and creating more sludge.
Now I am not a mechanic, but it would seem to me that something just got into your newly cleaned oil pan to create all that sludge. Ruling out the local neighborhood vandals, that would leave only gasoline, rainwater, or coolant, that could possibly gain access to the inside of your engine. My feeling is to always try the lowest cost shots first. Flush the radiator, replace with the proper coolant, Lexus specifies its particular type of coolant, make sure you bleed all the air out of the system and pressurize it, and then see if you get the sludge again. Also, take a look at what comes out of the radiator. MY 95 LS has 150k miles on it, and the engine runs smooth as silk, no reason yours should not either.
Good Luck.