http://egauges.com/vdo_send.asp?Sender=300F_150C&Cart=3

Which one do I need for a 05 sti

In other words, what is the thread and pitch of the OEM drain plug?

Hmm.. don't see anything big enough there...

If you *really* want to use the drain plug, that's going to be M20x1.5 I think

Alternatively you can use one of the gallery plugs (either the front location where the stock pressure sender is, or the rear 'blank' one) which are M18x1.5

Since the page you linked doesn't appear to list anything bigger than M16 I'd say your best bet is to go with the 1/8-27 NPT and tap an OEM plug yourself - my preference would be to get an OEM P/N 11024AA210 which is drilled and tapped to 1/8 BSPT (it is the adapter for the stock pressure sender) - that is close enough to 1/8 NPT that it can be re-tapped (you lose the top few threads, but the BSPT is tapered so that there's plenty of meat to re-tap further down). If you're not fitting an oil pressure, best to retain the stock sender in the front and use the rear one for your temp.

FWIW I *thought* VDO gauges were available with 1/8 BSPT senders - which would make it nice and easy - maybe not in the US of A?

I just decided to go with the WRXtra reolacement drain plug. Fussless http://www.wrxtra.com/cat_1.html

Also, temps up top seem to be much higher and have less of a temp range. So, if this unit were to want to protect against high oil temps from oil starvation, I would think the oil pan location is preferrential: that higher range of temps will only be evident when you are not in normal operating conditionsdue to oil than other heat generating items close to that galley plug.

Does that make sense or am I full of caca?