We've lived in older homes for many years now and anytime we've needed to do a quick paint touch up, 9/10 times it's oil paint. The bummer is, 9/10 times we're out of turpentine (to clean our brushes when finished), which causes us to leave that little touch up for later. But not anymore...
... dish soap!! It has the ability to be gentle on your brushes while removing the sticky, slick paint behind.
Try filling a small cup or jar with regular (non-foaming) dish soap so it is slightly deeper than the head of your brush. Mix your brush in the soap, coating as many bristles as possible. Remove brush and wipe on a paper towel or newspaper. The soap gives the paint something to stick to and will be left behind on the paper towel. Just repeat until your brush is clean!
Turpentine will always get your brush cleaner in the end, but it's a great way to save a brush if you don't have any around!