When or comes to what oil to use there’s always one general consensus. Whatever brand oil you use make sure it’s a motorcycle oil that is JASO-MA approved on the back of the bottle. And the recommended viscosity for your model KQ is 10W40. Unfortunately I’m not all familiar with all the other brands of motorcycle oils since all I’ve ever used is Suzuki brand oils. And since it’s Suzuki brand, it’s more expensive. For the alternative choice, I’m not much help there. But maybe someone else will chime in.
I get my oil filters from my local Suzuki dealer as well. So I don’t know of many alternative choice filters. But according to my web search of Mr. Cycles, the oil filter part number for your model 300 is: 16510-25C00. Take a look a the following diagram in the link I provided bellow. You’ll see part #13 which is the oil filter lists the part # I provided in the parts fiche bellow.
I personally use Suzuki brand oils. I’ve done so since ‘99 when I bought my 300 KQ. And I have 14,600 kilometres (9072 Miles) on it and it’s still going strong. No engine or transmission issues or leaks. Though if you have no Suzuki dealer near you or you just want to use a alternative brand oil then that’s perfectly fine. What brand you get however all depends on what’s available to you. The brand doesn’t really matter all that much but the type does. For your 250 what your essentially looking for is a oil that is JASO-MA approved on the back of the bottle. These oils are specifically designed for use with the wet clutch in your 250. Using oils that are not JASO-MA approved can or will cause premature clutch wear. So make sure you get the correct oil type and viscosity, (10w40).