Been there, done that. I've seen amps that have sat in restaurants for years that have a horrible goo made up of food grease and dust. Worst yet are any power amps with ventilation fans that suck that grease right in.
Lately I've been using a dollar store cleaner with the Amazing or Awesome brand name. Anything that will cut through the grease film is good. Just try and keep it off of the pc boards and components.