Castrol LMA Dot 4 is good, but I like to purge and flush with Ate Super Blue Racing fluid, which exceeds DOT 4 specs and is compatible with all other Dot 4 fluids and systems using DOT 4. The blue color makes it easy to see when fresh fluid is flowing from the bleeder

You can empty the MC reservoir at the beginning by draining it via the Left Front caliper, using the top bleeder and either of the two lower ones.

As you start at each wheel, open the bleeders and push the pistons back to expell as much old fluid as possible, then do the bleeding with a pressure system if possible.

I use the pressure port on my MityVac to pressurize the MC reservoir via a hose connected to a modified spare MC filler cap. It's surprising, but starting with 12 psi (gauge tee'd into hose), I can completely flush the Right Rear line and still have 10 psi in the reservoir.

Ther will be no "intermixing problems" using Ate fluid (check FCP Groton for availability). Also, FWIW, I believe the often repeated warning about using non-Castrol fluid in Girling calipers is based on myth—perhaps dating from the (pre-DOT spec) MG TC days, when the same was said about Castrol and Lockheed brakes (which may have been valid then).
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.