Well, if you want to do a complete job, you are supposed to drain the block. I didn't do this when I went from green to yellow. I got nervous about identifying the location of the drain plugs (apparently easily mistaken for important engine bolts). There was a lot of AF that came out from behind my water pump when removed after I thought I had drained everything via the lower hose. No apparent ill effects, 1.5 years later. But no doubt, draining the block is best, and I don't know about "compatibility" between the two.

Just a comment, I have essentially the same engine, etc - if you've never seen your car go over 80 C, I'll bet that your thermostat doesn't actually work - it's stuck open! This is a lot better than stuck closed, but the engine is not really running at an optimal temperature. I didn't go above 80 in the summer and not above 70 in the winter. I did major work, replaced the thermostat due to age and that I was already in there, and the temps went up to 85-95 C! Many people in these forums have observed this phenomenon with a new thermostat.

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86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

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