YAHOOOO!! Got it done but yes a royal PITA. I followed instructions from as many of the threads in this forum as I could find. Not much info was found for the xkr, but got the jist anyway.

I used a long hex 3/16" key to start undoing the s/c drain plug. Had to use vise grips on the key to get torque....hardly any movement allowed in between cooler lines and wire harness ( didn't remove any thing ..chicken ) ...JUST got it to release. Then used a long 3/16" socket hex key w/ball end and 3/8 drive ratchet wrench fed under rad hose from front of engine to allow (AT AN ANGLE) easier rotation of the plug until it dropped on to the paper towel dam I had put under the plug. Using a telescopic magnetic rod the plug was retrieved. Very little oil comes out.

Next..suck out oil...lots of patience required here as others have found. You can do this with gas line tubing attached to an inkjet refill syringe narrowing down to a wd 40 straw. I fished around the plug hole. Got lucky when poking down to right side & left side of hole. Almost 4" of the straw went in. I managed to get about 7oz. crappy smelling, coke coloured oil. only 31000 miles on the beast.
Took about 1/2 hr.

Filling was easier...put back about 7.5oz before it came out of the hole.
New Part # for GM s/c oil is 10953513, Old was #12345982 about 7.00 cdn per 4oz. bottle ...need 2.

Next : replacing the plug..#*&^% fiddle time trying to catch the first threads of the opening for the plug. Lots of light required to see down in there. Once it started I got out the long socket ..ball end key to spin it home...finish torque of plug with long straight key. Done..check for leaks after a spin around the block. or later too.