Does the same light in the dash light up fully if you short the oil pressure sender lead to ground? If not, it's likely the oil and charge bulbs got swapped at some point.
If the right bulb fires above, it's time to measure your canister depth and adjust the filter gaskets/shims accordingly.
Current: 93 R100GS, 03 XR650R
Prior: 73 CB350, 77 R100/7, 83 R100RT, 04 XR650L
I love every motorcycle I've owned and even some that I haven't.