I don't know what the recommended interval is, but I just change mine once every year or two. 30,000 miles is probably about as long as you want to go between changes, but since the level should be checked periodically, it's not much more difficult to go ahead and change it while you're checking.

Frank

'89 [color:"white"]G-Raider[color:"white"] [color:"black"]Supercharged 3.0L, MegaSquirt 2, lockup A/T, 2.5" exhaust, 172k, Cibie H4s/Oscar SCs, Hella Micro DE fogs, Cobra CB, Superwinch hubs, LSD rear/Aussie Locker front, Bilsteins, Lifeline AGM, Rust-Oleum