What kind of coolant did he put in your system, make sure it is the right stuff. Get either the MB coolant (which is a yellowish-orange) and pay more for it, or pick up some Zerex G-05 (same stuff but almost clear in color).

It is better to have mix, the proportions of which will be determined by how much freeze/boil protection you want/need; however, nearly everyone will do fine with a 50/50 mix. Your mechanic was right about one thing, you could go to northern Alaska, where temps can reach -50 F and you would be okay with 100% coolant. However, I think he probably just wanted to charge you more to fill 'er up with straight coolant.

The Zerex specs are as follows:

40% coolant = -12 F freeze point and 260 F boil point
50% coolant = -34 F freeze point and 265 F boil point
70%* coolant = -90 F freeze point and 277 F boil point**

*70% is the maximum protection point
**boil point assumes a 15 psi radiator cap

I just did a drain and flush on my system because the PO had just had a flush done in prep for the sale. Unfotunately there was 100% green stuff in there. Not only do you not want the green stuff (doesn't provide enough protection for diesels and the various metal types of the cooling system), but I also believe there might be some disadvantage to running straight coolant - don't recall what off the top of my head, maybe a pH issue.

You should just drain out about a gallon and save it for later (if it is the right stuff) and then add a gallon of distilled water.

__________________
'95 E300D ("Tank") - 231,000 miles
'79 240D ("Biscuit") - 197,250 miles (Sold)
'83 240D ("Ding-Ding") - 217,000 miles (Death by deer)
______________________________________

"Back off, man. I’m a scientist” ~ Peter Venkman