How many miles should you get out of the car before putting in oil.The people at the garage said we should put a litre of oil in for every 600 miles.Is this true?

that is absolute rubbish,a ltr for every 600 miles?you would be out a fortune on oil if that were the case because the 1st service interval is at 18k.Ive serviced megane diesels & found even at 18k without the customer lifting the lid & there still be half a sump remaining,Ive found that recommending oil consumption figures often puts people round the bend,best advice is to check the level regularly,like normal maintanance which is advised in the handbook,we see vehicles for 6k safety checks & this seems adequate.a few early new shape meganes had oil burning trouble but have now seem to be rectified in the newer models