I wouldn't worry about cracking lines by gelled WVO. There aren't many compounds that expand when they solidify (water being the most famous exception).

As far as knowing when it is safe to switch over, that depends a lot on ambient conditions, your system, etc., etc. The best thing to do is put a temperature gauge inline near your IP. When you first switchover, watch the temperature. If it is too low for your liking, switch back to diesel for a little while, then try again.

On my 300 SDL, I watch the temperature as well as the vacuum being pulled on the fuel system to monitor switchover conditions. If I flip the switch and the vacuum takes a dive (indicating the fuel pump is having to work real hard to pull more viscous oil through the lines), I switch back for a while. My temperature gauge is not reliable enough to use as a marker, I need to get a better one first.

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Keep everything as simple as possible-but no simpler--Albert Einstein