My 87, in the summer runs straight unheated WVO. If the temp doesn't fall below 65 I'm safe. This, of course, depends on the composition of the oil you burn. Oil with higher melt point fats (animal or vegetable) need a hotter climate to be burned without much heat. Fortunately for me my oil has little fat so I am comfortable running it this way. As the weather cools I will add D2 in increasing amounts up to about 40% in winter. Around here that generally means lows in the mid to upper 20's and highs in the 50's. The car has been run this way for the last 57K miles and is doing well.

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Muleears
'07 E320 Bluetec 133K my DD
'04 Jaguar XJ8 VDP, 34K
'10 Hyundai Accent 60K Grocery Getter
'02 VW Golf soon to be on the road again
'97 E300 Diesel Son's DD
'61 VERY tolerant wife

Hampton Roads, VA USA

Gone but not forgotten:
'67 250S 95K
'86 300SDL
'87 300D Turbo, 364K! R.I.P.
'98 E300 Turbodiesel, 213K
'02 S420, 164K
'01 Prius 138K