Always go with the recommended, but as a rule of thumb, go for as thinner oil as you can within recommendation.
Thin = flow = good.
TS doesnt burn oil like the JTS so you can get away with 10w40, however the JTS has a much higher compression ratio, and with it's direct injection it has much higher fuel pressure in the pots (60 bar i think) so you get more fuel dilution in the oil, and the oil rings aren't up to scratch..
for all those reasons, the 10w60 is more durable and minimizes oil consumption and it is more able to cope with fuel dilution (to a degree).
The down side is that for normal use it never gets up to operating temp, its too thick, so reduced flow (bad).. everything is a compromise. Some blame the famous exhaust cam wear on the fact that we are forced to use 10w60, but in my experience, this hasn't been the case - but I use top quality PAO/Ester blend stuff.... or, maybe I've been lucky.
The only time you would go thinner in this engine is if there is no sign, or little sign of excessive oil consumption.. then you might step down to a 10w50 and monitor the consumption.. if its fine, then a 10w40 - but I'd keep an eagle eye on how much the car burns oil - if there's signs of increased usage, go back to recommended grade.
Anyhoo - there's a top notch thread here
on all that is oil and stuff...