Lower Leg Oil for XFusion Mcqueens

I've a pair of XFusion McQueen Roughcut HLR forks that I've picked up for a Mullet HT experiment of a 150mm travel HT. They were set at 140mm, and I've dropped the travel to 120mm, but in the process I lost the oil out of the lowers.

I can't find much info on what the McQueens use, but I've seen some of the other models use 15ml of 7.5wt fully synthetic in each leg.

Anyone know what oil I should be using? As I seem to recall not all wt ratings are equal. I've some Castrol 10wt mineral fork oil, which I suspect I can't use as XFusion state fully synthetic. I've also some Rockshox 5w30 that I use in my Pikes, but I'm presuming that's too thin? The oil that came out looked much thicker than the gold 5w30 Rockshock oil, and was red.

Ideally I'd like something I can pick up from a local petrol station or similar, but I'll do mail order if I need to.

Cheers.