I have run Royal Purple for over 20 years( even sold it for a while years back )and it is my preferred product line for all vehicle fluids. The SL formula contained RP's special Synerlec additive( ZDDP based ). The SL oil was very good stuff.
OCI's with the old SL stuff were supposed to be good up to 12K according to RP. However, as with any oil it depends on the condition of the engine it is used in, how the engine is used( i.e. hard driving, towing, conservative, etc... ), and so on. I would say a safe run would be 10K. Without some UOA's I would not be comfortable going longer on any oil. I have seen the RP SL stuff run to 15K and have some life left according to a UOA. Again, 10K should be ok unless your engine has issues now.
Lots of folks on here who will say it is junk, over priced, etc... I disagree. It is an excellent product line in my experience.