Numbchux

That's the entire function of the oil cooler, it exchanges heat between the engine oil and coolant. Oil flows through it to the filter and then back through it to the engine block, and coolant flows through it via a pair of rubber hoses. The hoses are flexible, so you can unbolt the cooler from the block and move it around some (plenty for replacing it's Oring). One of those hoses goes to a steel line that crosses in front of the oil pan, this can be unbolted from the engine to gain even more flexibility without opening the cooling system. I'm not sure if it'll be enough to really be beneficial in accessing the oil switch. In 2008, Subaru switched coolant types. I've seen newer cars with conventional green in them, but I certainly wouldn't mix them, at which point a gallon of Subaru stuff typically makes more sense then flushing the system to replace with conventional green fluid. Adding fluid isn't any different. I think that setup still has a conventional radiator cap/overflow bottle.

I did some looking. That allen bolt is likely one of the upper oil pan bolts to the block. Probably could remove it without an issue, but shouldn't need to. Yea, make sure you're not using an impact socket. Only downside to removing the oil cooler, is in order to really get any more room, you'll probably have to drain the coolant system and disconnect those 2 hoses (easily flexible enough to replace the cooler oring itself...). And this is new enough to call for the Subaru blue coolant.

There was a TSB about the TPS on H6 models causing mild transmission issues without throwing a code. There is an updated part number from Subaru for it. It's not exactly cheap, so get your hands on the FSM testing page to test the resistance values, but that might be your answer. That's the only thing I can think of. The Transmission unit uses speed and throttle position to determine the appropriate gear. If it was getting bad speed info, you'd have other symptoms....so it almost has to be TPS. The TCU is a learning unit, so it might be worth doing a capacitive discharge (disconnect the battery for a few minutes, and step on the brake to use up any electricity in the capacitors) to reset things to a factory default. It may be learning some bad habits.

SOP for the 6-lug conversion is just to redrill the hub. So the brake parts are not changed. Should be standard EA82 parts.

It's well above the level in the pan. Barely a dribble, as I recall. Much easier to access with the oil filter off, though. I had a hell of a time getting the new one started with the filter in place. Also I had an aftermarket one on it which used a different sized socket.

It's completely possible to piece together without a donor car. And if you know your way around the cars, it can be an excellent way to do it. I have several times. BUT, I will still say that generally-speaking the best way (cheapest, lowest risk of failure, most complete) is a donor car. I'm in the middle of a Lexus 1UZ 4.0l DOHC V8 swap for my 4Runner. I bought the engine out of a car, and could not hear it run. So I bench ran it before I bought almost any other parts for it. Those ECUs are notorious for having failing capacitors, so I replaced those, and bench ran it again to make sure it still worked. Then I coated an exhaust manifold and ran it again. All together, it's probably run about an hour on the stand. Skip the first 10 minutes to hear it run.

This will depend very much on the local laws. Typically the state has a vaguely-worded statement about when and where you can drive it, which pretty much completely leaves it to the local law enforcement's discretion. When I got collector plates for my Brat and 4Runner (probably 8-10 years ago), I definitely had to sign something stating limited use (similar to the WA list posted above), when my dad did it for his 300ZX (more like 15 years ago), there was mention of mileage (although no inspection or anything to enforce it), when I did it for my Celica last year, there wasn't even any mention of it. The only requirement now, is to have another car registered with standard plates in my name. I kept standard plates on my Celica for years even though it was seasonal use, as I do daily drive it in the summer. I got pulled over one spring for expired tabs. When I told the officer that I bought them over the winter, but forgot to put them on when I got it out of storage, he suggested that I register it as a collector. My old neighbor, before I moved a few years ago, daily drove 2 cars both with collector plates year round. He did have a car in his name with regular plates, but his wife drove it. Never had a problem. I did get grief at the DMV/DNR office when I got an ORV sticker for my 4Runner. The lady at the counter, "You're going to take your collector bajaing through the woods? Your going to get caught!". I told her not to worry about it, and I've never had a problem. Of course, it's not a large savings. My insurance didn't change (I already take coverage off them when I'm not driving them). Tabs on older vehicles are only like $35 a year, and I don't have to run a front plate....neat.

Yea, it'll fit. Adapter plate to an EA82 transmission (better for offroad use), or use an EJ one (better for street use). Custom transmission crossmember. Some creativity/research on front axles. Wiring. Fuel Pump. Some Plumbing. It's been done many times, documented fairly well. I'd call it about a 2 or 3 out of 10 as far as engine swaps. But I'm pretty well-versed in swaps (currently putting a Lexus quad cam V8 in my 4Runner, and did a lot of work helping a friend put a 6.0 LS in his '98 Legacy).

The '06-'07s are ugly, and didn't sell well. The '08-'14s just look like an overgrown Outback. I frequently don't notice them.

This would be my recommendation. But, here's my stream-of-consciousness post about inner wheel seals: Short version, I ended up test-fitting a Timken/National 225678, and it looked like it would work (I don't like the XT6 knuckle/brake setup for my applications, so I never put it to use).

I knew I'd seen a similar discussion on this topic awhile ago, so I looked up the thread. Sounds like it might be the same car. Either way, no follow-up https://www.subaruoutback.org/forums/110-gen-2-2000-2004/494839-2003-subaru-outback-3l-h6-electrical-onstar-issue.html I haven't had any issue by unplugging modules. My '04 had Onstar, I unplugged it all and tossed it, and everything works. My '00 had a Subaru accessory shock sensor mounted next to the radio, tossed it, security system works as it should (no shock sensing, of course, but whatever). Ideally, you'd still have the stuff you cut out, but even without it, get the wiring diagrams (linked in the above thread), and repair what was cut, and that will probably fix your problem.

Good point, '00-'01 had a recall to spray a coating on it for that, but now 15 years later (I think the "recall" spray on my '00 was performed in '03), they're getting pretty nasty. They changed the coating in '02, so they're much less susceptible, but still worth looking at. It's possible to replace, but not a small job, and worth considering when buying.

Yea, very few "common" issues on the '00-'04 cars (by far my favorite Subaru). 4-cylinder cars are notorious for Head Gasket leaks, but unlike the stereotypical head gasket failure, these almost always manifest as an oil leak. Annoying, but as long as you check the oil regularly and don't let it run out, they'll run a long time (I had an '03 that was leaking when I bought it, and I fixed it right before selling it about 40k miles later). 6-cylinder cars have an issue with the serpentine belt pullies that can fail with little warning. Replace them preventatively. Depending on trim, Subaru started phasing in immobilizers into the '05-'09 cars. If that's the case, keep in mind a duplicate key is $150-200, and if you loose the last one, it's a HUGE project (make sure you always have a spare). Turbos were an option in this vintage, these are much more finicky about maintenance, I don't recommend them for daily use. Wiring into the tailgate is fairly common to fatigue were it bends between the gate/body. If it has any issues with the rear wiper, lock, or lights in the tailgate, there's a decent chance it needs repair there. Individual breaks can be traced and repaired, but it's about $80 for a new harness to reset the clock. But far more important is general maintenance. Fluids, matching tires, stuff like that. I normally look for low-mileage, neglected cars for cheap. Last year I bought a '00 with about 320k miles on it from an acquaintance who I know takes good care of his cars. It has been eye-opening how much fewer problems it has...

It's an interesting theory. But to me, it seems that if the surfaces in the joint really are that worn, it's going to be a problem either way. That said, I've brought some really troublesome joints back to life with new boots and grease. I've never bothered to swap side to side. I just re-boot originals.

Wow, thanks for the update. I never knew him, and don't remember the last time I saw him post, but he certainly was a regular in my early USMB days. Sorry to hear that.