Hi, I agree with this, when our big tank is freshly filled its a nice red colour, but as it goes down and after about 6-7 years its changed to a light brown.....
It may be heating fluid/kerosene/gas oil though, but as long as you say it feels oily, it will be fine. I use a 50:1 mix of heating oil/engine oil in a 3-phase generator, no probs, 10 years plus. Perkins diesel tho....
Up to you, but I would use it, albeit dilute it say 50:50 to give you confidence, it will save you half the fuel costs till it is done and it was evidently what the MOD ran it on!!
Alec.