Before a consonant on can use either version of the prepositions e/ex. Both seem to appear in prefixes as well, but ex- is often assimilated. It seems that, for example, words beginning with F take ex-: efficere, efferre, effundere… On the other hand, words beginning with L take e-: eligere, elevare, eloqui… My examples are verbs, but the pattern does not seem to be restricted to them.
This little observation suggests that some consonants (like L) require the prefix e- while others (like F) require ex-. Which ones take e- and which ones ex-? Are there perhaps consonants that can take either one? Is there an easy way to memorize or understand the division to two consonant types?