Outer Limits 4x4 Board

looking at running a full synthetic engine oil in my petrol 80s for something different but while the gregorys manual specifies 15w/40 or 20w/60 all the full synthetics are 0w/40 or 5w/50. is it ok to use these or is there a brand that is more suitable.

being twincam figured thinner oil would be better but is this too thin etc ?

also heading north (deserts etc) for some time so not sure if its ok in that situation.

thanks in advance, regards,
nathan

ps: search reveals much about oils but not this specific Q.

i would on run synthetic if specified only seen engines fail from running wrong type of oil.

all reports say stick with factory recommended so thats what il do.

thankyou.