Here is the long version! 3500 is what Susuki recomends
The engine on a 600 Katana is actually the old oil-cooled GSXR 750 motor with sleeves* (well, on the older engines; the late models have direct-bore cylinders with no sleeves) to choke it down to 600 CC displacement and a different set of valves & cams to change the engine behavior. Without changing the engine internals, the single biggest performance boost you can give your engine is switching to a good quality motor oil, such as Castrol
GPS synthetic motorcycle oils or Mobil 1 Racing 4T (formerly MX4T)
(doesn't void warrantee if using 10 to 20 weight rated oils in the API SF/SG categories). You'll notice the difference from most types of oil (we recommend driving both immediately before and immediately after the oil change to feel the difference).
To learn more about motorcycle motor oils, read: CyberPoet's How to understand and choose motorcycle motor oils
. *** DO NOT USE API SJ500, SL OR SM rated oils, nor any marked "Energy Conserving" in the lower half of the API ring in your Katana/GSXF! ***
For an even more substantial performance difference, you can try Castrol R4 SuperBike Oil (5W-40)
(or any other Ether-based 10w40/5w40), but the lighter nature of the oil may induce some pumping or lubrication problems at low RPM's when cold (when the oil pressure is at it's lowest, such as at idle), and it will probably void your warrantee (since it's lighter than Suzuki's recommended weights for the Katana; see Caution, below). That said, I ran R4 as the standard oil in one of my Kantana's for 12,250 miles without any issues as a result that I noticed (high speeds and in the Florida heat to boot!), but I will not recommend to anyone else for street usage and I may have worn my rings down some as a result of using it (got a new, younger Kat so I can't say what happened to that one after the 12.25k miles I ran the R4 -- just that it was good up till then). Castrol has just released R4 formulations in 10w40, 10w50 and 15w50 in the UK; if we're lucky, we'll get the same in the USA one of these days... CAUTION: Do not use Castrol R4 SuperBike oil during the engine break-in period. We recommend using traditional motorcycle dino oils (Kendell's 4-stroke motorcycle oil is supposedly very high in special compounds for the engine break-in period [anti-galling compounds] if I can find it, or I traditionally use Castrol Evo4), and changing them at least twice as often as recommended during the break-in procedure (as well as the oil filter). More often is better, and changing the oil on a new bike at 40 and 100 miles makes sense. The metal shavings generated during break-in scratching the various internal engine parts are the source of most long term performance losses that can be readily prevented. As it is, even well past break-in, I almost always change my oil every 2,800 to 3,000 miles (depending on the type of driving done, given Suzuki's recommendation of 3500 miles or sooner) and have never regretted