look in the mnaual, it should say the recommended weight for the 1zz-fe gearbox. in this case the nulon additive may not be suitable for his box or might not even be needed. hows the box been driven. has the oil ever been changed before? lots of factors. and that mechanic charging him that much, he hasn't even opened to box yet to find out whats wrong. best thing to do is get the recommended weight gearbox fluid from a shop and drop the oil. see if there are any metal bits coming out. i know for a fact that if it hasn't been changed the consistency will be close to water and would be dark as. i changed a mates oil on his stivo a little while back and he had travelled 140xxxkms without changing the oil once. the oil that came out was less than a litre (stivo C64 boxes take 2.3L) like water and was very very dark... luckily for him though, there wasn't any metal bits flowing out.