86 international s1700 dump

I do concrete for a living, I do the driving of the dump truck & mechanical repairs on our gas engines, my question is the truck is sluggish as hell & if its warm out like 65°+ it'll try stalling out when turning, braking, or even idling... Not sure what size engine but I know its a v8 diesel none turbo. Its a 5speed manual with the high/low box on the rear end(I kno because I jus broke it & my boss hadda have it in the shop for repair)... But I'm worried bout it stalling & hitting somebody... We use 15w-40 motor oil, not sure what get oil, I even tried cleaning the air filter

86 international s1700 dump