What type of viscosity is recommended for my 99 Seville STS. Also, I notice that my car burns alot of oil, is this common? and is it repairable?

Oil consumption is common with these engines. There is a complex procedure done by Cadillac to free up stuck rings. 10/30 I believe is recommended but check the owners manual.

10W-30 for pre-2000 Northstars.
As for the oil consumption, some people reduce it by doing a couple of full throttle runs with the shifter in 2nd. Of course do this in a safe location, but the high RPM shifts will help to free up any carbon from the rings, without having to go to the dealer for a chemical treatment that does the same thing.

The loaded full throttle runs have been somewhat unsuccessful, especailly if the condition has been long standing issue. Carbon builds up on the oil ring, from stop and start and " granny " style driving. Even Cadillac's non intrusive fix has been marginally effective, and many engines had to be rerung.

Thanks a bunch for the info and I will try it to see if it works.