Hello Mat & Scott,
My "Dealer launch" Rover 75 KV6 was built in May 1999 and I had an encounter with those oil cooler pipe/hose assemblies when renewing the timing belts. At the time the car was 19 years old.
Whilst the assemblies were not rusty and in very good condition (I have an undertray fitted), I can pass on some problems which you might be able to improve upon should you commission a production run.
1. The hose part of the assembly is extremely inflexible. With the oil cooler mounting bolts removed, I was unable to move the cooler sufficiently to provide access to the air conditioning compressor lower bolt (necessary for timing belt renewal). If it's possible to increase the hose's flexibility, that would be a bonus.
2. The cooler pipe unions where they meet the engine just above the oil filter are huge. A 22mm AF tool is required which is very difficult to manoeuvre in the restricted space available. If these unions can be reduced in size it would be an enormous advantage.
3. The 'O' rings fitted to the unions at the engine end I found to be not listed as a separate part. Fortunately my originals were in good condition and could be re-used but I'd say there's an unfulfilled need here.
To answer your question directly, my car will be 25 years old later this year and I don't expect to have to renew these oil cooler pipe/hose assemblies but who knows what the future holds?
Simon