Coolant flush, with Redline WaterWetter added to the mix. Confirmed with the local Mitsubishi dealer that they had a replacement oil pan drain plug in stock, so performed an oil flush with 0W30 synthetic, and then filled with Castrol Syntec (fully-synthetic) 5W50 and a ProTech oil filter. Noticed a little lifter noise; need to run it a bit to see if that remains.