I've never mixed blends on my Mustang, but I did do it on my dads 1993 Toyota Camry. I change the oil on all the cars in my family whenever I'm up in Niagara Falls visiting and I've switched between conventional oils to synthetic blends. Conventional oils I used during the spring and summer months and just before winter I'd switch it over to synthetic. Sometimes I'd put four parts Catrol conventional 10w-30 and 1 part 5W-50 of castrol synthetic. Never had a problem with leaks and we sold the car 2 years ago for $4500 with 300,000km or 187,500 miles on the engine (1.6 km = 1 mile).
I've heard the worst that can happen with switching to synthetic on a high mileage engine is that the synthetic will loosen up the old oil left by the conventional types on the oil pan walls and recycle it throught the engine. As for leaks, those were always there but plugged up from the old gunk from the conventional oils used. If you really want to do the switch yourself and do it properly you'll need to spend about $100 on 10 bottles of synthetic oil (your choice), 2 premium oil filters, and one bottle of internal engine cleaner. On your next oil change, take the car out for a nice long drive. Stop on the side of the road and pour the engine oil cleaner in. Drive the car again for about 15 minutes. Park it, and drain the oil and remove the filter. Let the car drain well. Then pop open a bottle of one of the sythetic oil cans and with the oil plug still removes, slowly pour the synthetic it. Looking under neith you should see the oil you just poured in come out with some extra gunk. Depending how much gunk you see, you may have to use another bottle. Put the pan plug in, tighten and put in a new oil filter. Fill up the car on the new synthetic oil. Take the car out again for another long run this time (Do shopping or something). Then go home and while hot, do another oil and filter change with the synthetic and you should be fine. Most guys here may say the 2 oil changes are unnecesary and you only need one, but that's pretty much what the professionals do and they charge about $100 for it to. It's safer too as you don't want any of that cleaner being left behind for 3500 miles and it ensures you got all the gunk out.
