'Grey Porridge' is a term used in the Classic Brit world to mean the everyday commuters, rather than the 'Rock Star' Vincent's, Goldies, Bonnies & the like. The Z400 4 stroke twin was, I think, the Oriental equivalent. Not that fast, fairly cheap to run, & usually reliable - although some early models could have head issues, I believe. Mine was a 1978 ex demo bike 18 months old, with 4.5k miles on the clock. I sold it 2 years later with 30k added on, last time I saw it there was nearly 90k & it was about to expire. Top end was an indicated 95, but I once saw 105 flat on the tank, racing a mate in his Dolomite. MPG - normally about 50, but I did have it drop to 30 battling against a strong headwind on the motorway. In those 20k miles what went wrong? The OE silencers rotted out, replaced with a Motad 2-1. Head & tail bulbs blew often, until I rubber mounted the units. A carb diaphragm holed. That was it. I sold it to a young lady who toured Europe on it. Two other friends had them, one went what is now called 'Adventure biking' all through Europe & into Africa on his with no trouble, the other had a lot of top end trouble on his earlier model.