I work in a shop, and I don't know what "Jiffy Lube" is like down there, but I do engine flushes all the time. I have never seen it cause a problem with an engine, and we refuse to flush engines where we know that the sludge is all that's holding it together lol. I'd probably recommend one for your vehicle bye the sounds of it, but what youe say sounds quite extensive. I pretty sure the tech was just trying to bs you. I've never heard of the trans fluid trick, but we use it to clean our floors on occasion, so why not, but it is a lot more viscous then engine oil. So, you might want to go with a less viscous oil, but no lighter then 10w30. As far as the rad flush comment up there. you have to change a car's coolant every 48000km (30 000miles?) because it contains antioxidants that prevent rust, though I think you'd be safe if you pissed in it (depending on how bad you had to go). I think that covers everything. Anymore questions about it, and just PM me and I'll hand over answers. Later guys.
Deep_Blue