Biodiesel blends higher than B20 can cause problems with natural rubber engine components, such as seals and hoses. Biodiesel will degrade rubber, so any seals or hoses in the fuel system that are made of rubber will be susceptible to damage.
Biodiesel blends of B20 or below should not cause problems with rubber components, but users should periodically check rubber components when using any Biodiesel blend to make sure they are not degrading or getting hard.
As the use of biodiesel increases, most equipment manufacturers are increasing the use Viton-based materials in seals and hoses. Viton is a synthetic rubber substitute that will not be damaged by biodiesel.
Most diesel engines built post-2002 will be protected. Many diesel engine manufacturers will now warranty their engines for B100 usage.