I can think of several reasons not to use 4-stroke motorcycle oil in an automotive application:
Significantly increased cost
Lack of friction modifiers
Refused warranty coverage and may actually void warranty
Use the right tool for the job (and this isn't it)
If you must add a boost to your "regular" car oil, buy an oil booster product (not recommended), or buy a quart of boutique oil with more Gp IV and Gp V base (e.g. Amsoil, Redline). Or just use the boutique oil instead of your regular stuff, it might be cheaper than 4-stroke oil.