I used much higher concentrations of rape and soya oil in my BMW powered Vauxhall Omega, and it saved me £5k over the years, all I had was a leaky pump by the time I scrapped it at 175,000 miles due to autobox problems.
I have used smaller concentrations in the VW as it is direct injection. I have never had any problems mixing it, I have done many tests at a variety of temps. It is fully miscible.
There are many websites that have the full S.P. and university test findings. Rapeseed oil is much better than some of the oils they have in other countries.