I have used Penrite 5W40 Diesel for the last 180,000 on the Defender and would not use anything else.
Just because a mechanic says so does not mean it is an absolute, just the way they have been trained, misinformed or trying to sell you oil.
As for the loss of oil, was the sump empty or just off the dipstick? Very unlikely for the oil itself to be the cause.

