I don’t understand it either. I finally went to the new high dollar low VOC pass thru jacketed line to see if I can get some durability.
I would think it would be cracking from the inside out if it was the diesel fuel. What I see is fine checking on the outside jacket followed by cracks on the end of the hose that progress further inside over time. I have split the line and don’t see checks in the middle of inner line. I don’t see where (max) 10% waste engine oil (mostly) or mineral based hydraulic fluid (rarely) should wipe it out. It lasts a few months.
It wouldn’t be so bad but the tank switch valve is even with the front of the side tank and so you can hardly get to the lines.
I’m retired so I don’t have access to any of the suppliers any more. You might try emailing the mfgr tech service if you can find an address off the net, and maybe include some pics.
If you can get past the gatekeeper and actually find an engineer or service guy they usually are interested in looking at failures.