It's an Internet hoax that has been around for a while and has recently reappeared. You can pretty much recognize the earmarks of a hoax by the fact that it sounds really alarming and makes you wonder how such dangers could possibly have been covered up. I knew that the warning you received was untrue, but I decided to check it out. If you go to Hoaxbusters, you'll find a listing of a number of Internet sites devoted to alerting the public to hoaxes and debunking misinformation. Spend a few minutes checking them out, and you'll no doubt recognize some e-mails you've received over the past few years.
Rapeseed oil contains potentially unhealthy amounts of erucic acid, a toxic fatty acid.
But canola oil has been bred to contain very low levels of this substance, so it's safe to use. What's more, canola oil is mostly monounsaturated and thus healthier than saturated or polyunsaturated oils, but I still consider it a distant runner-up to olive oil. Research has shown that populations that consume olive oil as a primary dietary fat have significantly lower rates of both heart disease and cancer than those that don't. But data isn't yet currently as strong for canola oil. I've always cautioned against buying most canola oils found in supermarkets.