The castrol magnetec 15W40 is THINNER than your 10W60, it may be getting too thin, especially after a blast, the thinner it is the more pressure drops.
I would try your 10W60 in it, as long as you are using FULLY synthetic it will be fine, mineral oils tend to break down way too easily.
The gauges are notoriously inaccurate, just put some decent 10W60, or some 15W50 mobil 1 in there and see what the gauge says, but don't worry anyway.
tabetha