This, as it was explained to me, happens when cold pockets form inside an engine. These pockets generate an acid that mixes with the oil and caused it to jell. Right now Toyota has a big problem with it. I know because my wife's mini-van with 20,000 miles on it and regular oil changes jelled. The engine had to be rebuilt. I asked them what they did to fix the problem and they told me that they were not sure that it was a problem. They felt it was due to owner neglect. I showed the engine compartment of my '73 that I happened to drive to pick up the van and asked if he thought I was the type of person who neglected anything automotive! I took the van and traded it for a Jeep the next day. The wife got my Denali. She's happy, I'm happy and Toyota is not!