To flush or not to flush...
 

What do you guys think about an engine flush for a high mileage car? Would it clean out the old sludgy oil and varnish deposits and make the car run better and last longer or would it loosen up the old sludgy oil and varnish deposits and cause them to plug up the oil galleries resulting in a ruined engine?
Is there another way to clean out an old engine short of tearing it down and rebuilding it?
What kind of experiences with this have you had?

that crap will only cause more issues go to a DR and ask for something to clean out your arteries they will look at you and ask you are u looking for a heart attack?
ALl it will serve to to loosen up stuff and clog it where its most needed.

Not to do +1

So is there an alternative? Frequent changes with high-detergent oil? Tear-down and re-build? Sea-foam?

There is no cure for sludge already there. Only a tear down.

When I was 16 my dad got me a very well worn 1972 Chevelle that had a 350 motor in it and tons of miles on it. That engine had the worst sludge I have ever seen in my life and I'm now over 40 years old. I just changed the oil every 3,000 miles in that car and my dad would not let me use a cleansing product on that engine for fear it might damage it. I drove that car for two years and we sold the car to a guy who said he was buying it just for the engine. Two years later we seen his wife driving the car... I have read about engines seizing in Europe due to a sludge problem they had over there years ago. I was going to buy a Lexus ES 300 a few months ago and I found out on the Lexus forum that some of those cars had sludge problems due to Lexus recommended 7,000 mile oil change intervals in the owners manual for certain years. Apparently people who used conventional oil and 7,000 mile oil change intervals for years sludgged up their engines in the Lexus cars.

I would not recommend a cleansing product on the engine either. I recently switched both of my personal cars over to Pennzoil Ultra class synthetic motor oil and a Mobile One extended performance oil filter. Ferrari USA said it kept their new engines cleaner and closest to factory clean better than any oil they tested. That oil would probably clean out an engine to some degree after using it and changing the oil a few times.

I did quite a bunch of research on the bobistheoilguy forums and listened to my friends who prefer synthetic oil in their cars. I only recently switched over to synthetic oil two months ago. Ferrari and bobistheoilguy forums convinced me that Pennzoil Ultra synthetic was the one I wanted to use. I always used conventional oil and always changed my oil every 3,000 miles. That always worked extremely well for me over the years. I have never blown an engine regardless of how many miles were on the car.

Not to flush. Use good oil and change it regularly and you won't have anything that needs flushing.