The only system that's a chain I've not personally seen fail is a the US small block type V8s where the chain only goes from the cam to crank, all the other systems have tensioners or guides that fail.

Think about it, a change system has all the same guides and idlers and tensioners as a belt and some are oil PSI activated and have o rings that can (and always do) fail. Some are over reliant on oil to prevent wear (GM 3.6 and and ford V8) and at the end of the day the timing system will not last as long as a belt system that you change out.

Again the only two systems I've seen are gear drive and double row roller chains that do not have any guides at all that don't fail or develop play. Belt systems do better in any instance outside of that provided you change them, and on the LS430 book time is 3.7 hours and having done a few it's one of if not THE easiest belt out there.

The UZ V8 has many engine over 1000000 miles without any rebuild because the timing components do not wear out. Other engines simply don't make it that far without play developing and while they might run if you floor it and get aggressive they will die quickly. Chain slap on higher mileage engines is a very real problem that is ignored until it jumps a tooth and ends up with me for a valve replacement and all the guides replaced.

You are absolutely correct why a belt was used, at the time plastic and polymer technology was not advanced enough to successfully create a guide system for a timing chain assembly that has several feet of contact and friction unlike 4 cylinder engines of the time that have a much simpler chain path with way less friction and contact overall. V8s that are overhead cam, quad cam especially, have enormous amounts of chain that would've been physically impossible to make guides for that last 150-200k miles.

There is an entire aftermarket segment for the ford 5.0 for billet guides to avoid the well known chain issues those engines have when performance driven.

 

When I bought mine it had 154k, my mechanic said it looked pristine, but I too couldn't find any proof it was done. This is an interference engine so it's not worth the risk. As others have said how it looks don't matter one bit.

Make sure you replace the seals, water pump, idlers, and tensioner as well.

 

I'd have them change that and the water pump or have it discounted on the purchase price.

 

It appears Lexus agrees with you: https://lexus2.custhelp.com/app/answ...lt-replaced%3F

What is the factory recommendation to have the timing belt replaced?

Our timing belts have been very reliable. However, the timing belt is subjected to severe stress if the vehicle is operated under extensive idling or low speed driving for long distances, such as in heavy commercial use like delivery, taxi or patrol car. If the timing belt breaks, the engine will stop running and will not restart until it is repaired. While it is possible that the original timing belt may last the life of the above engines, for peace-of-mind, the timing belt may be replaced every 90,000 miles. Also, if major engine work is performed, such as a valve job or engine rebuild, the timing belt should be replaced.