OK ... if Allison M20's, they'll be WT 40 ... so same spec as engine oil ... single grade WT 40 ... DO NOT ATTEMPT MULTI GRADE !!!
Engines do require single grade oil, as does the Allison's... with multi-grade, you'll probably head for an engine seizure sooner rather than later... so PLEASE check that you do carry the right oil onboard at outset as very few fuelling points en route will carry straight WT 40 oil in stock!!!