Oil tends to blow out of turbos when they are old and enter the air intake system. This then clogs in the intercooler as the lowest point in the system. Land Rover suggest flushing every 48,000 miles. What also happens is oil leaks past the intercooler top hose, runs down the intercooler fins and they start to gum up with dust and debris blown in through the radiator grille. If you imagine 20% of your intercooler fins get clogged then that's 20% less cooling of the air and a subsequent drop in power
I used "Gunk" to flush the intercooler, but all sorts of degreaser are adequate. Afterwards I dried it inside for an evening on the radiator just to make sure all traces were gone before starting the engine.
