Lubricious is the prettier of the two variants, with a sound that suggests its meaning, so we prefer it. Both variants come with adverbs created by simply adding -ly to the end. The noun for lubricious is lubricity. Both forms are related to the verb lubricate, hence the implication of oiliness.

This is the adjective to use in describing eels, snails, jelly-fish, throats, eyeballs, and things of similar texture: "Mary Jo then placed upon the table a bowl of boiled okra so lubricious the diners had trouble holding it on their forks." However, this word works just as well describing things that are figuratively slippery: "Phil Anders asked Marian Kine to his apartment with such a lubricious smile on his face that she called a cab to take her home."

Lubricous, which could have resulted from the omission of the I (i) before the suffix -ous or simply an adjustment of the word back toward its Latin ancestor, lubricus "slippery". The Latin word comes from a Proto-Indo-European word, sleubh- "to slip", with an initial Fickle S. As usual, the Fickle S was lost in Latin but remained in the Germanic languages, so sleubh- went on to become sleeve in English, something we slip our arms into. English slip and slop share the same origin.