You can use "flushing oil".

Not sure where you can buy this, but we used to use it all the time. It was a product made by Castrol (I suspect all manufacturers have it) and we basically tested an oil, then ran the "flusher" through, and then went onto the next oil. This was with dyno testing.

The flushing oil was slippery, but couldn't be used under too much load 'cos the shear strength was pretty low (I'm told), and it contained a detergent of some sort.

I've actually seen it in the local shop here in Thailand, so it's for sale somewhere.