Comingling Fuels
Just in case questions regarding compability of various heavy fuel oils should arise, pls find below a recognized method to determine compability.
The spot test procedure is received from FOBAS. 
Compatibility testing of two (or more) fuels:
a) The fuels in question are to be mixed in equal parts and then a single drop of the mixed fuels to be placed on the filter paper.
 (If no FOBAS filter paper for spot test is available, then filter paper from your boiler test kit can be used) ,
 (Alternatively Coffee filter 1 layer can be used)
b) The filter paper with the drop on to be heated in an oven at 90 - 100 degr.C  for about 15 -20 minutes.
c) After the oven treatment compare the filter paper with the attached comparison chart with photos of 5 spots.
    Spots like 1 & 2 is acceptable, whereas 3, 4 & 5 is unacceptable.