I was just looking through the service manual...We have sodium filled exhaust valves...This is used for dissipating heat. Exhaust valves do tend to get hot...but I'm not sure they get THAT hot. I'm speculating that the spring pressure is WAY up there to get these engines to turn 6,000 RPM. Higher force = higher heat. With high spring pressures the "tappets" tend to bleed down. This is especially true when the oil is hot and has a lower viscosity.

Just a thought.

BTW...my 2005 ticks a little too!