I’ve had two lorries with Gardner engines, one a Seddon Atkinson 400 with a 180, the other a Foden 8 wheeler with the ‘uprated’ 201hp engine, they were both heaps of junk that wouldn’t pull a greasy stick out of a dog’s arse and I hated them both with a passion. I have no idea about oil consumption, as I never checked it in the hope that the gutless wonders would self destruct 
I did have the one in the Sed Ak run backwards on me though. I was on the north side of the Woolwich Ferry, as you join the pier there is a bit of a hump. I was almost on top of it and when the queue moved forward I let out the clutch, but nearly stalled it, so I dipped the clutch and gave it a bootful of revs, then all hell broke loose, the thing was screaming its nuts off and blowing blue smoke out through the air filter. I put it in gear and went hurtling backwards 
Had I known then what I know now, I would’ve left it to blow itself to pieces, but like a fool I stalled the engine and it went on to live another day, many days in fact 
I’ve read the comments about oil consumption, wow, a gallon a day, that is some serious wear to the cylinders/rings. Common practice by engine manufacturers today is to rebuild if oil consumption is more than a gallon every 3500 miles. (the actual figure is worked out on oil use compared to fuel use, but a gallon every 3500 miles gives a rough idea) I think the problem that the Gardner engines had was that they never got hot enough when new to allow the rings to seat properly, therefore oil was escaping past the rings and blowing out the exhaust. I can’t comment on reliabilty issues, mine were both very reliable, too bloody reliable in fact 
As you can guess, I’m not a huge fan of the Percy, but I have to say they did make a lovely noise, especially those straight eights 
So what led to the end of Gardner engines, in a word…Arrogance