I've been trying to buy a cleaning product to run through my Bronco's engine, to clear out some of the old oil sludge and varnish that's accumulated (1969 engine that was rebuilt probably 15 years ago). I've been good with oil and filter changes, but it has had a lot of very slow, hot, dusty rides while I'm four wheeling, so I don't doubt I've had an accumulation of crud. I've did a fair amount of research and came across something that sounds just like what I'm looking for, and it numerous recommendations from people who have used it.
I order some to try, and low and behold, the order was rejected because I live in Kalifornia. All other states, this stuff is legal, here they must think it causes earthquakes or something, but they do allow it in CA for manufacturing. Sure, I bet it's somewhat nasty (lots of esters), but if it cleans out the crap from my engine and it runs better, isn't that a good thing? The esters do evaporate, but the crap it dissolves in my engine is trapped by the oil filter, which goes to my local HAZMAT collection.
Part of the terms and conditions for buying the product through the manufacturer (only source of this stuff):
"In California, this product cannot be sold to an individual consumer. By placing this order, Purchaser certifies and warrants that all products purchased through this website will be used for industrial purposes in the manufacturing process only and not for retail sale, personal or consumer use."