Elevate the front end on ramps or jack stands. You can get to the drain plug from underneath. Forget WD40, use Liquid Wrench or PB Blaster. A 19mm wrench or socket will do the trick. BTW, the drain plug doesn't come out, it's a petcock.
It you simply can't loosen it, try this. Either open the drian valve or the feeder hose from the expansion tank at the bottom passenger side of the radiator. Remove the thermostat housing and the t-stat. Make sure the heater is on. Insert a hose into the t-stat housing and flush until it runs clear.
You may need to lower the front end to let gravity help drain the block. To refill: raise the front end and pour cooland mix into the t-stat housing until it overflows. Replace the t-stat and housing. then, fill through the rcovery tank the rest of the way. Start the engine and top off after the t-stat opens.
91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)