Monitor, and if seeping continues around the edge of the cap, I would certainly open the fill vent, put a pan under for the leakage, loosen the cap and replace the o-ring, at a minimum. Depending on cap condition, you may need to replace it, as well, tighten to the proper torque [from memory only, 25nm?]. Sounds like an ill-fitted o-ring.

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2006 X5 3.0 6-spd w/Evo UUC ssk
sport/premium pkgs
born Valentine's Day, 2006.