Engine oil type?

I have heard that you should never switch to a synthetic oil if the motor has been broken in, and has used a conventional oil for several years. Is this true. My next question is my truck, 2002 4x4 Z-71 chevy silverado.5.3 V-8 automatic, has been running 10w40 valvaline for years. It was purchased from my father in law. I was wondering if castrol high mileage 5w30 would be a wise switch, but my neighbor, with a mid 90's ford V-8 4x4 said that he had tried castrol high mileage, and that it was a synthetic blend, and his truck developed major oil leaks. His mechanic stated that the synthetic blend had weakend the seals, and told him to run a straight 15(i think) weight oil for a short time, drain it and go back to a convential 10w40 oil. Is any of this story true, and is the high mileage oils offered 2-day
synthetic blends, will they hurt my truck if I swithch, or help? And is it even a good idea to switch from what I am running now? I know a lot of questions, but I am so confused with all the oils available 2-day. Want the best for my truck. Any answers would be appriciated. Thanks, and god bless.