Sludge I have seen (and analyzed) out of several Toyotas was the direct result of:
1) Extended mileage/time specified by the manufacturer between oil changes.
2) Driving only short trips not warming up the engine to its normal operating temperature.
Manufacturers have a tendency to extend the oil change intervals to try and convince owners their car require less normal maintenance than the competition.
These extended oil change intervals are generally specified for use under "ideal conditions" i.e: Highway driving at posted speed limit in temperate weather, very little stop and go traffic, engine always up to optimum operating temperature, light load, gentle driver.
If you don't meet ALL of the above criteria, replace oil at much shorter intervals.
I'd rather have my engine bath in a fresh API certified cheap recycled oil than in a 8,000mi old top of the line expensive super totally synthetic oil.
JackD