The infamous sludge problem confirmed by Toyota pertains to 5S-FE as well.
Toyota stated engines 97-01 covered. Which brings a question - what fundamentally
different changed in 1996 5S-FE engines which contributed to this problem?
One thing I see in 1997 - on Camrys - the oil cooler disapeared. Is it still in
97-99 celicas? What else is so different?

i dont know what changed, but i can tell you how to prevent it.
change your oil & filter on the reccomended intervals.

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Well, people did it - and still got it.

belive it or not, but years ago i got a letter from toyota telling me about this problem. they even said that if the motor was to ever fail from the sludge issue, they would cover it for a certain amount of years.

all the 6th gen GT's had oil coolers stock.
heres a pic of my valvetrain from when i swapped the '98 motor in (yes, i love showing off this pic! haha)

no chance of sludge buildup when the motor is maintained like that!