Oil weights

[jakriz]

I've been using Mobil 20/50 oil for 9 years now, but just yesterday I tried some Penrite 5/40 Synthetic. My oil pressure guage will not register, only the slightest bit when cold & although it stayed cool just driving around on the street, I changed it back to the Mobil 20/50 when I got home & my oil guage instantly came to life again.
Anyone else had these experiences??? Another problem is that I have a sponsership lined up for next year with Penrite & I wouldn't feel right telling them that their oil is crap for my little beetle?
Should I just find out what other weights they have or should I just stick what I know works??
regards
Jak