to be safe try a Dell OEM fan or similar Foxconn fan like that from HP.  You do not really need a fancy fan for chassis in this model.  The fan is supposed to spin at constant speed 100% of time. 8500 does not have temp sensor to detect chassis temp and change chassis fan speed. I think the header is 3 pin only.
I would like to replace my chassis fan or, at least have one on hand before the original one fails. I tried a bgears replacement but I got the fan failure message on bootup and the fan would not stay running beyond post. It is a 3 pin plug that goes to the mobo which the replacement was. Dell only seems to have 120mm fans available. I need a 90/92mm. Cannot find a Dell compatible anywhere. Thanks!
XPS 8500, chassis fan replacement?