I have the spoiler mounted on my car. The oil temp does goes up. My friend has one too and had the same problem but it sure does keep the rain water out. Both cars do not have external oil cooler except for a 3.5qt sump on my friend.
The Blue Bug is mine with 1600DP engine. My friend's Pink Beauty has a 2275cc engine. Note the oil temp when driven on highway with the spoiler, without it the oil temp won't go past 220F
just want to share my bug here, I'm surely inspired by those beetle from Jeffrey's VW Pro Shop from Japan, here's mine, a 1302-S with Personal steering wheel, RAZO pedal's and shift knob, 1679cc blue printed, MSD ignitiong system, 4-2-1 stainless steel headers with custom made muffler, 16x7 Enkei racing series rims with Yokohama AVS ES100, custom front strut with adjustable camber plate, strut bar at front, sway bar at rear, urethane bushings and Shachs gas charge shock absorbers, and other stuff...BTW I'm from Philippines.