A heat transfer oil, comprising: a. an isomerized Fischer-Tropsch derived lubricating base oil having: i. greater than 10 weight percent and less than 70 weight percent total molecules with cycloparaffinic functionality, ii. at least 23.6 wt % 1-unsaturations by FIMS, and iii. a ratio of weight percent molecules with monocycloparaffinic functionality to weight percent molecules with multicycloparaffinic functionality greater than 15; and b. less than 0.2 wt % of an antifoam agent; wherein the heat transfer oil has an auto ignition temperature greater than 329° C. (625° F.). Also, a process to prepare the heat transfer oil, comprising: a. selecting the isomerized Fischer-Tropsch derived lubricating base oil having the defined properties; and b. blending the isomerized Fischer-Tropsch derived lubricating base oil with less than 0.2 wt % antifoam agent to prepare a heat transfer oil having an auto ignition temperature greater than 329° C. (625° F.).