Hello all,
I can only write about the Augusta sliding pillar suspension, but the one on the Aprilia is similar.
IMHO the oil at the bottom should be engine oil like the oil on the top (shock absorber) chamber. The oil in the bottom chamber not only serves to lubricate the ball thrust bearing below the springs but also to lubricate the lower guide and the lower sliding cylinder( king-pin). As those parts are above the nominal oil level, the only way oil gets there is by splashing. The oil splashes up to the lower guides by the movements of the front wheels on the uneven (mostly) road surface. I think that’s quite a good system as more the road surface is uneven, the more splashing is effected.
Therefore, the oil at the bottom chamber should be thin enough to splash up.
I hope that helps
Karl