On Friday, October 12, 2018 at 8:36:35 AM UTC-4, wrote:
"The grease has dried out..."

I guess now would be a good time to dispel this old husband's tale.

When the union between heatsink and transistor is undisturbed, dried out grease that is like cement is perfectly fine.

You beat me to it. It's not the liquid component of heat sink compound that transfer heat, it's the solids that are suspended in the goop. The compound may get hard, but the solids are still packed between gaps doing their job.