Diesel 2.5 vm engine running 5w/40 oil.

[si-h]

The owner of my taxi has informed me that from now on, the 2008 cab with 250k on the clock, will use fully synthetic 5w/40 oil. Over the past two nights, 7 litres of Asda's finest oil has smoked out of the exhaust....... An oil consumption never seen on this cab ! 1L per week normally.
Even thought the manufacturer states this is the correct oil, am I correct in arguing this oil recommendation only applies to a new engine, or an engine that has always used this oil.
Am I correct in saying it is way too thin, and way too late to start using this oil ?
Any technical wise words of wisdom to support my stance ?