My mechanic just put peak long life in my 2001 grand marquis that originally took traditional green...will this stuff harm my engine in any way even though it says compatible for use in all cars and trucks regardless of make model or year and mixes with any color antifreeze. I have been reading conflicting things on the net about this....I just don't want any problems.

If he just added coolant make sure that he added same color coolant. Because if he mixed red one with green it might cause to stuck holes of head core! I recommend you to drain whole coolant and refill with red color coolant (which is more environment friendly). Do same thing every 3 years. I hope that will help for your question...

as said above if it was all replaced thats fine..also try and add 1 bottle of WATER WETTER to the water/antifreeze..k

Do not use the red colored crap in your Mercury. If the mechanic mixed different types of coolant I would flush and refill with the proper coolant. There is no way to tell if the mixed coolant will damage anything, most likely it won't. Also there is zero need to put any additives in a 50/50 mixture of coolant and distilled water.

Ford has it's own brand of red coolant which is different from red coolant found in most retail locations. Motorcraft coolant.