switching to synthedic oil on high mileage truck

I have a 1994 f 150 with 115000 miles on it. I have run casteroil 10-w40 in it from the day I bought it new. It runs great dosent use oil between changes,it has a 5.0 in it.
My brother wants me to switch to a synthetic oil but I have heard you can have problems with varnish and dirt etc. breaking loose if you change to a synthetic after all this time. Is this true? Any experiance with this? Thanks rickyc

From my experiences it can be true. My dad's friend had always used normal oil in his Pontiac LeMans and he switched to Mobil 1 and it ran really shitty he said that it broke a lot of gunk up. He's since switched back to normal oil and hasn't had any problems.

However my neighbor switched to Mobil 1 with close to 100k miles on his car and he had no problems. So I guess it just depends on the engine.