My bike's history
1996 GS
bought it with about 1500 miles last year. Currently at 8k miles. Bike sat for like 8 years rarely ridden at all. Fast foward a bunch of repairs (gas tank carbs fuel lines etc..)

Problem: Burning a qt of oil in a 1000 miles. I commute on the interstate a lot (80mph) 90% of the time. Sometimes i go 2 hours at 80mph when i go out of town which is about every other weekend. I changed to synthetic oil at 7k miles and have put in a qt since (at 8100 miles now) Doesnt smoke on start or idling. Friend says it smokes when i shift gears when im on it hard (I usually go to 9-10 if i am on it hard).

Should i switch oils to motorcycle or thicker oil?  Or is it an engine problem?

Try a thicker oil.

I use a Synthetic 10W-60 but you could use a thicker mineral oil if you prefer 

I'd stop using synth and I'd use 10~40 dino.  If she still burns then use 20~50.

If her rings are failing you should be able to redline her on the side stand and watch the puff of smoke.  If valves are allowing blow-by and such there would be a bit of smoke all the time and a lot on start up.

Switch back to Dino Oil.If you use Synthetic Oil too soon before the Break In Period Ends your Rings won't Seat.The Only way to get the Rings to seat is to Hone the Cylinders and try Again.
Even at 1 Quart Per 1000 miles you won't Notice that much Smoke.It could just be that the Rings may be Sticking in the Grooves a Little Due to Carbon Build up or your Valve seals could be Leaking a Wee Bit.

Anyway Just Switch back to Dino Oil and try that for a 1000 Miles and see what Happens.

It's typically an air-cooled engine problem.  Tolerances are loose to allow for greater expansion.  At high RPM, it tends to burn a significant amount. 

If the rings are stuck, then sometimes a bit of ATF in each cylinder and allow it to soak overnight can break them lose. 

Valve seals can be rejuvenated somewhat with an oil additive or something like MaxLife (which has the additives in it) that cause seals to swell. 

Thicker oil will burn slower.  Dino less than synthetic.

If you're using a lot of oil there's not much reason to use synthetic. 20w50 will probally help.

Bikes with few miles on them that are stored for long periods of time can develop rust on the cylinders, sometimes siezing the engine. You probally have some pitting on the cylinders and your rings may be stuck.

The best thing to do would be to pull the cylinders and hone them and then install new rings. You could just run it for a while though with the thicker oil and see if it clears up.

syn oil mith on break in is a complete myth. all VW/audi car now come with it form the factory 5w 40 castrol syn

i would run 20w 50 and see what happens but it sounds liek the rings my be going, to test that do a compresion test, they should be with in 20 psi of each other lower than 140 and higher than 60 psi

holy schnikies!  The Srinath hacked Ranger's account!

My old GS would burn about a quart every 1k, no big deal. I'd take it in on interstate for hours and just top it off once in a while. I'd say that for higher mileage GS's 1 quart per 1k miles is about the rule of thumb. I always used 10-40 mineral oil fyi.

-Ash