A&J, nice to be back, just very busy in real life. I agree with you and Yvan, using OPC at higher dilution rates is a good thing to do. It uses less product as most of the time leather will not get as dirty as wheels or engines so a weaker dilution is more appropriate.
Not sure when this was posted but it does talk about interior cleaning at 3:1 http://optimumpolytech.blogspot.co.uk/2015/06/power-clean.html