The suzuki service  manual list SE or SF grade 10w30 as  being usable (if you can't find 10w40 ) from -20c/-4f to 30c/86f deg - have a heart and pour in at least 10w40 dino oil - of course 10w40 or 20w50 motorcycle syn mobil 1 is my choice.

Castrol GTX 20-50 has been around for a long while.  Used it in my '67 Spitfire back in '71.  Drove to work.  Unknown to me I split a radiator hose and the water all drained out while I was parked.  Going home I depart downtown on what is now the 110, past the 10 traffic lightens and I accelerate, de-clutch to shift up and the engine STOPS.  Keep the clutch in, pull to the side (across 3 lanes).  Find no water, engine so hot the pistons had expanded and siezed in the cylinders.  Let it cool, start up and get off the freeway and get some water.  Drive home.  Fix hose, change oil.  No appearent engine damage.  The oil did it's job of protecting the engine.

I like Castro GTX.

I bought my Savage at Suzuki City in Biloxi MS in Feb 98. If you were to call there and talk to Mike, he would tell you to run 20W50 year round in south MS.

If you get the chance, go to the Little Sturgis (MS) rally this year. If it still happens... You won't regret it. It is co-hosted by the Christian Motorcyclists Association and Southern Coast Riders Motorcycle Association, Gulfport Chapter. Cleanest, sanest fun you can have on two wheels. I liked it better than the big rally in Sturgis, SD.

There IS a Castrol GTX motorcycle 20W50 as well as dino GTX 20W50. I run car dino, my particular scoot likes it better. 10 weight oils are too thin for Dixieland riding conditions. Trust me, I rode my Savage in MS, AL, FL, LA, TN, KY, AR.

Ride the wheels off it. If you get to Gulfport around the 4th of July, the SCR chapter used to host a rally at the Copa Casino. Good mudbugs, decent vendors, nice ride in bike show.
-WD