Hello,
I have a 1981 300SD with 245k miles driven often and nearly 100% highway use. Recently (6 months) I've been having to add coolant. I have to add usually 2-3 quarts per 2 tanks of fuel. I cannot find any external leaks, it only seems to lose coolant while running. I've never had any issues with overheating (solid at 82 degrees, occasionally climbs to 85 on a long hill), have no coolant in the oil, and although my exhaust is more visible than I'd like it doesn't smell like coolant. Expansion tank was new in 2013 and hoses new last year. It's worth noting that the PO had a used head put on the car at 200k miles after an overheating incident. (Found out after purchase by looking at receipts).

The car runs excellent otherwise, so I'd like some advice.
-Is it dangerous to keep running it in this condition?
-How is it possible to have a leak but not an overheating condition or water in the oil?
-Is a head replacement a relatively garage friendly repair? I'm not short of normal tools but don't have many M.Benz special tools.

Any other advice appreciated!
Thanks

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-1981 300SD 242k, stock - sold @ 250k
-1994 Toyota Pickup 4x4 R150F 5 Speed, 5VZ V6 swap.
-2004 Volvo V70-R 2.5T