You do not need any sort of special attachment to your grease gun to use oil in
it - just fill it with oil and use it just the same. A little may leak out
where it clips to the nipple, but not a lot, and no more than the amount of
grease that ususally escapes when you do this sort of job. Oil, being
thinner, may have a better chance of squeezing out of the sides of the joint -
but, being thinner, it flows into the trunnion much more easily than grease,
so you don't need to put so much load on the grease gun handle, so there is
less pressure trying to force the oil round the edges of the nipple.
I would advise you to get a new grease gun for oiling trunnions - you don't
want to have keep emptying and cleaning your grease gun every time you swap
between greasing and oiling. I picked up the cheapest one in Halfords for a
very few quid and it has worked fine.
You can jack up the whole of the rear of the car. However, I would advise
placing the jack under the rear cross-member of the chassis, rather than the
differential - I have done this many times. Remember to chock the front
wheels!!! This is less important if you have a trolley jack, but you should
really still do it.
Richard & Daffy