I bet AN6 or AN4 is what is used by STS in their kits, or close to that. AN10 is a 5/8" hose, very big.
Presumably the oil will not be way too hot when the turbo oil pump stops each time, so I'm not sure what the coking issue would be either.
I think the oil level sensor is not likely to work right if it was placed outside the pan(by a "T" fitting). I'd find a way to stop the error message, grounding the sensor wire etc(what ever it takes). The hole in the pan is very large, it's around a 1/2" NPT in size, I've read it somewhere. That is a source of oil leaking, so be careful with the sensor and seal components. Treat them as delicate so you don't end up with a leak later when you reinstall it.
Back to the oil return line, if it is placed like STS suggests, up at the top, then there will be oil in that line constantly, when off. I don't see a problem with the oil being in the line(what causes the coking), but of course you don't want oil to drain back to the turbo, or have gravity cause any to seep out of the turbo seals.
The way to do that would be to minimize how much return line oil is above the turbo. Mount the turbo higher, make the return line on the smaller side(less volume), use a check valve near the turbo/pump, and route the line upwards above the turbo as close to the turbo as possible(minimizes oil that can run back to it).
Routing the return line above the oil pan oil level is just to keep the oil from flowing out by gravity(if the port is below the oil level).