On an otherwise good engine I would NEVER use an engine flush. Especially on a
engine with hydraulic lifters. There is just too much chance for crud to get
broken loose and wind up giving you lifter problems.

The best way to get your engine cleaner is just change the oil more often.

If you feel the engine needs to be cleaned remove any parts you can and clean
them by hand, on the waterboxer this is just the valve covers, rockers and push
rods. This way any dirt gets removed from the engine and not send on a trip thru
it.