I have ran as much as 25% with out any problems. A little harder to start. My truck is harder to start than most even on straight diesel. I wish I had a second tank so I could use a higher % of oil in it and one tank for straight diesel during start up and shut down during winter. My biggest issue is finding time to filter the oil. I run it through whole house water filters. Seems to work and haven't had any issues. I have only went through about 20 gallons. I'm new to this and don't do it on every tank. Kinda lazy I guess.
2009 Suzuki LTZ400 2004 Mach 1
1993 Acclaim 3.0 1992 F250 7.5L
1989 F250 7.3L 1986 Honda ATC
1983 Capri 5.8L