The performance of diesel engines fail for a plethora of reasons, without codes it's hard to judge. Some fail due to MAF failures and default to ECU fuelling maps due to a lack of air, some are due to the use of bio fuel oil, some are just old and knackered or lack of fundamental maintenance but without codes and seeing/hearing the truck in question everything is a mere guess. I own and run an efficient TD5 turbo diesel but it's keeping it running efficiently that tests ones mettle. There are many 'specialists' out there but knowing who is b-llsh!tting is the difficult job, tuned diesels sacrifice certain aspects, drivability, longevity, etc, etc. there is no easy answer, similarly with petrol engines.

at higher altitude you get lesser oxygen so there will be loss of horsepower.

"at higher altitude you get lesser oxygen so there will be loss of horsepower."

I realize this, at some point however the ECU senses and adjust to the elevation/oxygen level right?

Unfortunately the mini vans passing me on the pass did not have the same loss of horsepower.

I don't recall it being so dramatic in the past but have noticed it before.

So does anyone have similar experience and is there a method to actually have horsepower climbing a hill in a V8 Petrol Disco?

The ECU normally adjusts the fuel to air ratio to cope with altitudes but obviously there is a limit/ceiling to some altitude heights it can adjust to.