Change the oil every 2000 miles, as recommended by GM in truck operators manuals.
With a filter, you should be able to go more miles between oil changes, but the consevative approach would be to change at the same 2000-3000 mile intervals.
When/if you ever want to add a filter, original-style by-pass canister filters can be eaily found. Jim Carter now sells a modern-style remote filter-head for a spin-on filter, AND it looks like it has been adapted to work as a by-pass filter (it probably has a restriction in the input port of the filter head).
