The NA Miata's power steering reservoir is known to overflow under high loads such as spirited driving or track driving. If the power steering reservoir is coated in a oily like substance, it is most likely leaking out from the cap due to excess pressure from thermal expansion.
Maruha has come out with a solution to cure this problem with their own Power Steering Reservoir Cover. It is designed to withstand the increased pressure of thermal expansion during heavy use.
Keep your powering steering fluid in the reservoir and not all over your engine bay!