And the failure was ....

The oil cooler!
It's a Flyin' Miata oil cooler kit I picked up second hand (but barely used) some years back, and installed by Automotive Plus because at the time I wasn't very confident on tools. I've always thought this thing was a liability where it's hung, at the bottom of the steering rack. It's taken a few scrapes from bottoming out on mountain runs where sudden dips in the road came up, and there appears to be a decent dent on the lower left corner. I don't recall any recent impacts that would have caused the damage, but the cooler was completely soaked, and once I pulled it off the car it was empty.
It's been in the back of my mind to remove the cooler for a couple of years. Well, that finally happened on Saturday.
Complicating the job, I also have a Thompson Oil Filter relocation kit, which sits on top of the sandwich plate to the oil cooler. As usual I made the whole job harder than need be.
Adaptors on adaptorsI pulled the relocation kit and lines, making a good mess in the process. Then the lines to the oil cooler, making even more mess. Then I flubbed around trying to unscrew the sandwich plate, but it fouled on the OEM oil warmer coolant hose barbs. I couldn't work out how it could even get on with those in the way. I finally found my print out of the FM instructions, the sandwich plate is held on by a post extender that threads on to the oil filter post, basically a bolt. Much easier to get off than I was making it out to be!
Finally with everything off I just put the oil filter back on the block and degreased everything. I don't think I'll put the relocation back in, it's extra fail points, and I can go back to using the stock filter location just fine. I won't bother with a replacement oil cooler, I don't think its' really necessary, at least not for the driving I do.
I haven't done a test yet to check nothing else is leaking, I was going to on Sunday, but Brisbane was absolutely tropical on Sunday after a deluge of rain Saturday and the sun was out Sunday morning making for an extremely humid climate, and it was just too much for me to be rolling around on the garage floor.