I've only cut a couple times with mine and wiped it clean after both occasions. As for oiling, I put on a light coat, didn't overdue it.
The rust/discoloration patches occurred a few inches below the kisaki and about 4"-5" up from the tsuba. They appeared on both sides of the blade as well.
Was wondering if this could be some kind of reaction to whatever glue was used to afix the two halves of the saya together.
Anything is possible of course, but over the years with Bugei's stuff I've not seen that. We've had virtually zero rust problems with their swords. I have seen a few folk who've had rust problems start years later, but it's really difficult sometimes to pinpoint why that might happen. Also keep in mind that all sorts of things can cause problems. One customer of mine had recurring problems with rust along the mune of his sword. The problem turned out to be how he did noto (resheathing). It turned out his skin oil was massively corrosive. And even though he thought he cleaned it well the reality was that if *any* was left behind for any time (especially on hot days) it would rust almost instantly. I pointed out to him that where he was getting the rust is exactly where he touches the piece so I had him rub another blade I had (unpolished) on his hand and put it on my rack. Just to show him I rubbed another area on my skin. His area rusted in a day. Mine didn't. Some peoples' body chemistry is also vastly more nasty. And those who use some lotions also have more problems.
But sometimes it just is what it is. Heck, I just helped out a guy who had a sword that was mounted probably 100 years ago that he's had for 20 years that just started a similar problem in one area. So 80 years of no problems. Then suddenly... Something is in the saya. How it got there, why it waited until now, etc. are all questions I can't answer. And the customer tells me it sits on a rack on his shelf untouched other than a cleaning every few months. As of now my only suggestion to him is offer to make him a shirasaya or a replacement saya for it. Shrug.
Carbon steel "likes" to rust. And depending on environment, location, etc. it will. People who live by the ocean in particular have many more problems fwiw. People in hot dry areas have few (although their mounts tent to dry out and crack). Nature of the beast.