I still have this owners manual for the GN that shows a graph, showing oil weights and temperatures and whats recommended for whatever climate. In california climate, a straight 30 weight covers anywhere from 50 degree weather to 110 degrees.
But thats relevant to the hypereutectic pistons they used from the factory which dont require thermal expansion to fill up the piston to wall clearance...since those pistons dont expand at all, you can hone the bores to .001" clearance, and have a really clean burning, non oil consuming motor. But so many people out there go with forged pistons which require more clearance since they expand so much. So at warm up temps, cold climates, and cold running engines (160 thermostats), you still have all this piston to wall clearance that hasnt been taken up by thermal expansion....and the pistons rock, the skirts crush and scuff, you have alot of blowby, the rings never properly seat, etc. Good machinists will always reference main/rod clearances by the book, so thats not an issue, and they arent as affected by operating temps. But when people go with forged pistons, theres new problems to deal with. You run an oil weight thats good for the tight rod and main clearances (TR clearances need to be tight from high cylinder pressures at low rpm), but is now wrong for a big piston to wall clearance, which shouldnt be there had the cylinders been honed to a size with operating temperature in mind. So if you run forged pistons, have the machinist run a tighter clearance on the cylinders....like .0025 to .003 and never higher, if you plan on running a 160 thermostat. Then you'll have a happy motor, when running the factory recommended weight. The manufacturers of the forged pistons want to make sure that whether you run at 160, or all the way up to 230 degrees, your pistons wont expand so much that they lock up or severely scuff....so they give this big margin of error to cover their butts. If you have your cooling system up to par, run tight clearances and you'll have a happy motor that burns clean, breaks the rings in properly and has little leakdown. I saw most forged piston manufacturers recommending up to a .005-.006" clearance on a 4" bore, yet I NEVER ran that much clearance when I honed the bores out. I usually kept it .002-.003 if they ran a cool thermostat....and I never had 1 block ever come back to me with problems. My old bosses race boat had a 427, and he had .008" clearance with his forged pistons, and the motor had terrible blowby and didnt run nearly as strong as the other boats, and when I did a quick re-ring and hone job on that block, his cylinders were out of round by .008"...!...which I couldnt fix cause he was cheap and wouldnt get the right pistons to fit a bigger bore. The cylinders were out of round because the pistons were rocking from excessive clearance, and the skirts were wearing the cylinders into an oval...and he was running a cold thermostat.......
You really need to know your piston to wall clearance to know for sure what weight to run, but in any normal climate, a 10-30 will do. With hypers and a tight p/w clearance, you could run a 5w-20, but that doesnt have the sheer strength to handle the rod and main pressures with the high combustion pressures from boost....so find a nice middle ground. If you hear a knock at all in the motor, down low inside, up the weight.
I would leave the thirty in if it gives you good oil pressure. My personal philosophy is to run the lightest oil that gives me the pressure I want warm. Actually, I should say that's the philosophy I subscribe to as it was someone much smarter than myself that came up with it. 
This ensures maximum volume across the bearings and therefore maximum cooling while maintaining adequate pressure.
