Personally, I don't agree with the following explanations:
"shearing due to the valvetrain design"
"smaller engines in tighter places"
"small oil sump capacity"
"change to SJ oil in 1996"
Take my puny 3 quart oil pan capacity 1989 Corolla, it has all
of the above, but this high revving 4A-F engine is famous for standing
up to 10,000+ mile dino oil drain intervals. In fact, the owners manual states
to change the oil and filter every 10,000 miles under normal driving conditions
and 5,000 miles under severe condition !! I have seen the insides
of many of these popular '88-'96 4A / 7A series motors in auto
wrecking yards with 200,000 - 410,000 miles on them and never saw
sludge.