oil comparison
 

I thought I would post an interesting experience I had this summer with an oil change on my '82 911sc. Took the car in for service to change oil and replace the cat with a bypass pipe (inside of cat was fried out). I have been running shell rotella t 15-40w oil at the advice of my mechanic. Since it was hot August weather and had heard so much good about Kendall 20-50w, I tried it. I noticed right away driving home that I was seeing higher temps than I had ever seen on the oil guage (needle was close to half way). Figured it could not be the bypass pipe so switched back to the shell rotella the next week and on a similar temperature day when I picked the car up noticed that it did indeed run cooler (just above first white stick on guage).

Question about bypass pipes: Mechanic installed a B&B steel pipe. Should I have installed a Fabiano Euro pipe with the bulge or does it matter? Thanks!

I'm suspicious that the oil switch could make any difference in temperature, especially the large swing you state.

Both, Shell Rotella and Kendall are high quality, mineral oils. The difference in weights should not be an issue either.
As a matter of fact, I'm presently using Kendall 20w-50 in my motor. Previously have used Castrol GTX and Mobil 1. There was never a discernable difference in engine temperature with any of them.

While I don't doubt you may have seen different readings on the gauge, I think some other factor is at work.