I'd dump and refill with fresh oil and a new filter after 1500 miles, but there's probably no harm in leaving it in longer. It would seem to me that if there's no appreciable change in 1500 miles, there's not much point in letting it stay in the mix.... if there is a change, it's best to get whatever dissolved gunk that has been removed - out of the engine.

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