I have placed 110K miles on my 99 RX300 without this problem...knock on wood.
However, my previous car that I purchsed from mom, a 93 Camry with the same 3.0 V6 had the sludge problem. I realized it when white smoke poored out the tail pipe when I hit the gas & I took it in to get looked at. This car only had 30K miles on it.
It was obvious to everyone when the valve covers where pulled off. The mechanic did his best to clean the engine as best as possible. I then put another 30K miles on it before I sold it with no problems.
My theory based on my mothers driving habbits and talking with a couple of mechanics is that she did not drive the car far enough to heat the engine completely, she just drove short trips. She said she changed the oil regularly, so I think the sludge is due to oil not heating up completely building up the residue.
Regards,
Rex
