What happens when you mix synthetic and mineral oil?

[thagannn]

The maintenance light on my speedster 150 has been on for the past 20 hours since we've got it back from the shop so we figured they forgot to reset it since the 150 hr check up. It was low on oil so I put in some 5W-30 and I just went to check the oil again and the cap was off and oil was spilled in the engine compartment. I figured there was too much pressure in the tank but I have no idea why it happened. It's the 255 supercharged model so I know it's not supposed to have synthetic, but I think it might've anyways and I'm afraid I mixed the two.