I was surfing the Internets today and found this site, wanting you to buy their steel tank processor plans. Only they do it by scaring potential customers into thinking that using a polypropylene tank is DANGEROUS:

Melt Down

Which is total bull****.

I mean, really. As long as you are conscious of safety (mindfull of where and how you place heating elements), you will be just fine.

I love how they issue the warning and then offer their plans for sale RIGHT AFTER.

Wow.

I have been using a 80 gallon processor for over fourteen months now, and ALL tanks are poly. I have routinely run the processor tank at 135° F during processing and have never had a problem. It is still going strong.

 

i bet they sell a lot of those by scare tactics like that. 135 is cold enough that you'll be fine forever in plastic tanks. as you said, its only a problem if your heating element is too close to the wall of the tank.
that said, a steel tank is probably more forgiving of ignorance - but anybody who can't keep an eye on where they place the heater shouldn't be processing that much oil in their home anyway