Toyota's engine was not thier high-performance engine. It
was their jack of all trades engine. They modified it to
improve oil circulation to fix the problem.

I found a site that lists these as primary causes of sludge
(I was surprised at the number of possible causes):

Faulty head gasket, allowing coolant to leak into oil system

Unburned gasoline contaminating oil

Inadequate oil circulation

Infrequent changes of oil and filter

Poor quality or nondetergent oil

Oxides of nitrogen from emissions system contaminate oil

Engine runs too cool in spots, causing condensation buildup inside crankcase

Engine runs too hot in spots, causing oil to break down faster

Seal on oil filler cap fails, allowing moisture to enter engine through valve cover

Too many short trips or frequent driving under adverse conditions