Any grease or oil will improve thermal conductivity substantially. A tube of clear silicone grease from the NAPA auto store is probably as good as any other clear grease, but it's the loading with various fillers that really improves the stuff and makes it "thermal grease". I would never put a clear grease in my projects, only something like Dow 540 (going from memory on the number), or Thermalloy grease. The auto stores usually sell white thermal grease for use with sensors, and there's nothing wrong with that either.