Kick inside,

Knowing you can handle our obsolete English units, I offer this. This method works with 850/S70 but there are small differences for the 6 Cyl S80.

Power flushing is a simple DIY job. I use synthetic Mobil-1 ATF or
Amsoil. You will need up to 15 quarts.
Disconnect the upper cooling hose. Open the metal clamp carefully and
wiggle, wiggle, pull, pull. The oil will come out of the hose end when the engine runs. Clamp 6 ft of clear plastic 1/2 inch ID hose (hardware store item) over
the hose end and prepare to drain into gallon bottles or a calibrated
bucket, so that you know when you have removed about 12 quarts. Idle
the engine to pump out fluid. Shut off immediately when bubbles appear
in the clear hose. Add two quarts down the dipstick hole using a tranny
filler funnel and hose from auto supply or chain store. Repeat the
pump/fill cycle. Somewhere in the middle of this you may run the shift
thru D N R while idling to assist the clean out. Once you've removed 12
quarts, the fluid should start running clean shortly thereafter, at
which point reinsert the cooling hose into the radiator. Top off the level per the owner's manual, being careful not to overfill.