See if you can get a 5W-40. I use this in all my cars and are quite happy.
The gearboxes I use Castrol BOT130M. It is a full synthetic gearbox oil with specs of 75W-90 and GL4. It definitely improves shifting. Was recommended by most guys on our local VW forum who do allot of circuit racing and drag racing.

Im going to use castrol magnatec 10w-40 in my CA, i think penrite make a 10w-50 which would be good if your going to do alot of hard driving as it has that extra 10 if the motor does get a bit hot and for summer months
Gearbox wise im gunna use nulon smooth shift 75w-90 in my stumpy , generally most nissan gearboxes will take this weight of gear oil

i use valvoline synpower 10w-50 in my landcruiser

very good stuff i reckon!

what oils should i get for CA18DET and skyline stumpy?

I tend to stick with Penrite as much as possible, anyone know off the top of their head which ones are recommended? otherwise i will search later tonight