So I had a look under the car last night and found that the cross member, right in the middle section, is wet (oil). I also found the back of the sump is damp with oil, but the gearbox underside appears to be dry. The wet patch appears to be forward of the steering rack.

I'm just looking for suggestions on what it might be (which leads to how much it's going to cost me $$).

Am I correct in almost ruling out the rear main seal?

Pics attached.

Attached Thumbnails

     

Is it engine oil or ps fluid? The first pic has black drop which looks like engine oil

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Doing it for all the wagons out there

I'm fairly certain its engine oil. It's black in colour, yep.

Where I mentioned steering rack, it was just to paint a picture as to the location of where the oil is accumulating.

That's pretty much where the rear main is, or as likely the problem the oil baffle cover(which if its the original is plastic which warps and leaks.)

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MY99 WRX EJ2.35L 720whp 9.48@147mph
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Have you tried cleaning the area down to see if it reappears? When this happened to me I realised I had been clumsy with oil top ups and it had run down the filler neck and then worked it's way to the back of the block....

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That's what I was worried about.

Haven't done that yet... but i'll give it a go.

Other things I've thought it might be is turbo oil return line, or split PCV line. Are these possible?

Ive given it a quick wipe, it's engine oil.

Interesting - the cross member is damp with oil from the passenger side all the way across to the driver side (and dirt and shit has stuck to it). What would cause that?

Oil spreads out. Its what it does. So it might start in one spot, but it will 'wick' and spread as much as it can.

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Doing it for all the wagons out there

Just an update...

Had the rocker cover gasket/seals replaced as they were leaking anyway, and the cross member was cleaned so I could keep an eye out for new oil.

After a short drive, there is a distinct drip on the driver side of the cross member, just a few inches to the right of centre.

Whilst driving and when coming to a stop, there is a bit of smoke coming from the turbo area, I'm hoping its just oil that may have fallen onto the up-pipe when the rocker cover gasket was changed, but its odd that its all happening on that side of the engine... ?

Update:

Smoke was just degreaser burning off.
Replacing rocker cover seal/gaskets helped.
Small distinct drip of oil on drivers side was loose clamp on turbo oil return line.