Drew,
I wouldn't worry about your corolla, they are pretty sludge free from the 4af series and up. I have a 5sfe camry 4cyl and it seems to be ok, though I do get worried. My friend who is a master mechanic at toyota said that the 6cyl camry engine is the main trouble maker, said the intake plenum is too close to the back of the engine thus cause a hot insulated pocket for the engine and a cold spot from the intake plenum. This causes the sludge, a friend has the old 4cyl 3sfe engine 2.0 liter, 1989 Camry and has had it for 258,000mi while changing the oil whenever..WAY beyond 3-5K/oil change. One time i went to do the oil change for them, and saw the dipstick at low (BLACK oil) and 1/4" of sludge under the cap.
Looked in the valve cover and just saw black. But hey it's still running and WELL!!
BUt back to your corolla, haven't heard or seen a sludge problem with the corolla, I believe the old 4afe/4aGE engines can take some abuse with lack of oil changes, not sure about the brand new 1.8liter ones.