I did wonder, i've never seen a standard half size metric O ring in a catalogue or kit either, but there's some weird stuff around too.
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That's the first hit on my google search: FKM O-ring 11.5 x 2.5mm Minimum 5 pcs - OringsandMore available all over Europe too but can't see in Australia [bighmmm]
I'll pop into the engineering supplies/bearing service tomorrow and hit them up for all three sizes.
One will fit! [thumbsupbig]
I want to do a dump and refill plus filter change on the weekend and take an oil sample after losing nearly 6 litres when the fitting came loose last weekend.