We have 10w40 in our cars.
. It's suppose to hit over 25 degrees celsius up here in Vancouver this summer though. Will 10w40 still be ok?
With Synthetic or semi-synthetic you can drop 1-2 weights for a given temp range and not worry. It is much more stable.
Which I think is part of the whole leak myth. Many when going synthetic change to much thinner oil.
I have run 10w30from 0 F. (-17 C.) up to about 105 F. (40deg. C.) without problems. I would not try that on dino though.
Dino I use 10w30 in winter, 10w40 in summer (same temp range as above).