Mud

 Mud  is the thing that all Grouches love almost as much as they love trash  & everything trashy. Oscar the Grouch usually has  Mud man  deliver his orders of Mud to his  trash can.

& since Grouches can't stand to be clean, they tend to bathe in Mud, not just collect it. Oscar the Grouch has stated that Worms, including his worm,  Slimey, enjoy playin in Mud as well as takin bathes in it.

Sesame Street

 * Episode 0297 (1971): When Oscar puts up "NO" signs all over his  trash can, the  Mud man  sees a "No Salesmen" sign & refuses to sell  Oscar  his fresh supply of mud.
 * Episode 0329 (1972): When Oscar  doesn't need any mud, the  Mud man  tries to sell some mud to Tom, who only needs dirt & water. After the  Mud man  tells Tom that dirt & water mixed together is mud, Tom buys 2 quarts.

Books

 * The Sesame Street 1, 2, 3 Storybook (1973)
 *  Big Bird 's Busy Book (1975) ("Front & Back Masks")
 * Who's Who on Sesame Street (1977)
 * The Sesame Street Bedtime Storybook (1978)
 * The Sesame Street Dictionary (1980)
 * A Day in the Life of  Oscar the Grouch  (1981)
 *  Oscar 's Rotten Birthday (1981)
 * City (1982)