My back brake is feeling a bit spongy (more spongy than when I got it) so I want to bleed it.

Do I have to go and get some expensive Shimano fluid or can I replace the fluid with the dot 4 or dot 5.1 fluid I already have?

I'm pretty sure XT brakes are mineral oil only!

NO!!

must be mineral oil

any mineral oil, LHM or Cyclo from halfords.

OK, thanks.

Next question (I've been googling while you've been answering). Do I have to use a syringe and inject from the caliper end, or can I do it the way I do on my old Hopes and pour the fluid into the reservoir on the brake lever?

I bled my XT brakes last night from reservoir down and they are fine.

Can be done top up,with syringe or top down via reservoir.
Cyclo/Halfords mineral oil is a good alternative to Shimano mineral oil.
About a fiver a bottle.

Cheers guys.

Think I can sort that now.

This stuff.

+1 for LHM