My understanding is that fabric softener reacts with things in the washing to form a nasty gunge that clogs up the machine. So never use it.
Not long after fabric softener launched on the market here, there were complaints of people getting 'gunk' and marks on their washing.
There was even a Fair Go program about it.
The waxy/silcon buildup in washing machines was called 'crud'. I think it was made worse by cooler washes allowing build up in the machine that would then eventually transfer to the washing.
I found that fabric softeners seem overly scented as well and residue(and perfumes) can sometimes cause issues for people with sensitive skin.
Liquid detergent works well when washing from cold to hot temperatures and tends to leave no visible residue.
Powdered detergents ideally need a little bit of warmth to help them disolve - but I reckon a clear liquid probably has a head start for leaving less residue on clothing.