In what way we're they troublesome?
Oddly enough, replacement diesel powered 80 series injector washers are aluminium.
I pressumed it was a cost thing.
Copper seals were too hard, and after a while they could leak allowing blow by which caused carbon to form in the oil which would find its way down to the oil pick, eventually blocking the oil pick filter causing engine destruction in extreme cases, obviously not helped when people insist on leaving engine oil in too long believing all this modern long life servicing bollocks, i learned long ago when i worked for one of the best maintained lorry fleets in the country, that oil is cheap and engines gearboxes and axles are not.
The alloy seals being softer crush down to form a perfect seal.
Many vehicles were re-fitted by Toyota with alloy seals, and certainly any early 3.0 litre engines (i'm unsure if the 2.5 litre Hilux engine had or has this problem) that needed new injectors were fitted with alloy seals, they could rattle after a while, mine did and were replaced FOC by Toyota just before i sold the vehicle.
Toyota in the UK raised the warranty on affected engines to 100k and 7 years if this issue caused engine failure, whatever people think of Toyota its impossible to dispute they regard their customers and their own reputation as important, and stand by their products, obviously if an engine hadn't seen any new oil for 50k miles they wouldn't be so keen to pick the tab up and i wouldn't blame them.
You can check VIN to see if your engine is one of the affected vehicles, but the recall is now long past, so anyone worried would be advised to get the seals changed anyway, or do as i do and inspect the pick up every oil change.
On 120 series with this engine you can see the oil pick up which is directly above the sump drain plug, so its good practice to let the oil drain overnight and have a good butchers at the filter before refilling, i do this when the weather is warmish...must admit i slipped in a quick oil change just before Christmas on a rare dry day but didn't bother looking this time, well i did look but not for too long due to oil drips.
As i recall the sump isn't the same design on Hilux so pick up not visible, whether an endoscope could be used to inspect the pick up i don't know.
Personally, and this is just me, i've shifted over to using Diesel specific engine oil, EXOL 10w40 for anyone interested, due to the higher detergent content of DEO, to me thats belt and braces that if an injector seal was to spring a leak then my twice yearly oil change regime using DEO would (hopefully) help prevent the oil becoming too carbonised to cause issues.
In practice, since i've been using this oil i notice the oil is staying cleaner for longer after changing, getting on for 1000 miles now before turning black on a 107k mile engine, even at 1000 miles i can still see through to the dipstick markings...yes i know this is hardly a scientific control test, but it suggests to me that DEO has cleaned up the engine internals during its use.
There's a brilliant Youtuber well worth watching, Fourby4Diesel is a mechanic who specialises in Prados with the 1kd engine, there's literally nothing he doesn't know about these engines, well worth a peruse on his channel for anyone interested in learning more, wish we had a go to guru like this in Britain, and if there ios i'd be obliged for someone to pass on details, i'd like diagnostics done on mine to see what condition my own injectors are in.
Sorry about all the waffle, its not a one sentence issue.