Rubber Preservative--HELP

[Eric Junger]

All,
I picked up a dated 1943 USMC raincoat at a garage sale on Sat. Its in great condition with only 1 small tear at the bottom---hardly even notice. Its a little stiff but does not appear to be cracking yet. Does anyone have a recommendation on how to treat it so the rubberized material stays soft and does not dry out and crack?
Thanks for any assistance.
Semper Fi,