old engine?

hi guys

ok went to the citroen dealership as usual to make an appointment for an oil change (skoda dealer is 2 busy)..... and asked if they have the castrol slx 0w30 and they asked me the mileage on the car....it's 113.000km (about 70.000 miles) .... and they said the engine (TDI 90 HP) is old for this oil as it is 2 thin for a used engine..... i have been using this oil every winter since now (the car is 2 and 1/2 years old).... and had it's oil change every 7.000 km (4.400 miles).

summer time oil has been either castrol 10w60 RS or castrol TXT 5x40.

are these guys right?

and second : is the castrol 80w90 API GL4 good for the gearbox? .... last year i had it changed @ the skoda dealership and have no idea what they put in ....just know it had the vw/audi/skoda/seat badge on it.

is there a specification that it had to meet (the gearbox oil)?

thanks