What makes you think it's low? Is it cavatating?
The filler on my 87 TD had a long neck, and the fluid had a ton of expansion room. I can't think of any reason for it to ever get low, as even a small leak, since its under pressure, would empty it pretty quickly.
If I recall on my 6.9, it too offers plenty of expansion room.

Chances seem pretty likely that you can drive it until you have time to get the correct fluid.

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes