[DeTomaso] synthetic gear oil

[Panteratime at aol.com]

Folks,
I believe last time I changed my ZF gear oil I used Mobil 1 75w90  synthetic. 
 I heard once you use synthetic you need to stay with  it.  Motor oil also.  
Any truth here?   Otherwise I'm going  with Valvoline 75w90 LS non synthetic
Thanks 



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