Got a couple tips about diesel into the crankcase to clean the sludge out. Also heard horror stories about all the moving parts rattle when the varnish goes away and every seal leaks and when the sludge goes away. What's the worst that can happen and what's the best cleaner if I choose to use cleaner? Motor is a stock 400, mileage UNK, good bit of sludge under the valve covers, lots of blue smoke when running, no big leaks. Thanks.

Not a great idea to use a crankcase cleaner for all the reasons mentioned above. The worst problems that I've seen when a pile of loose varnish chips end up in the oil pan and clog the oil pump pick-up screen.

Best bet is to remove and rebuild. If the engine is a sludgy mess inside, it probably needs more than a quick bath!

Blue smoke is bad, bad news. It sounds like you may have cracked a ring. I think the sludge might be the least of your problems.

I'd try cleaning it out first. Nothing to lose, right? Commercial cleaner (doesn't work real good) Diesel, or even kerosene for the tough stuff. Put it in hot, let it IDLE for half an hour, then change the oil and FILTER 3 times, idling awhile in between. Do it all the same day, keep it hot, and keep your fingers crossed!