Dear hevans1944,
Thank you for helping out !
Yes, I would agree with you on the DC702, DC704, DC705 diffusion pump oil availability in the market and their price tags.
Anyway, I am going to try that out in my diffusion pump (I found from scrap yard), using PMX-200 (poly-dimethyl-siloxane) PDMS, I am going to use 50cSt and see it's performance. I know that this silicone oil comes with different viscosity (5 cSt, 10, cSt ... 1000cSt..)
I read from online research journal, that their molecular weight is highly dependant on their viscosity. (50 cSt) silicone oil probably has 2000-3000 g mol^-1
50cSt silicone oil by chance , it has 250 °C boiling point ( 0.5 mm Hg). 100 cSt silicone has even higher boiling point (0.5 mmHg), but I am not sure how much °C
50 cSt also has good surface tension (cohesion forces) properties (roughly 23 dynes/cm) , not too bad compared to dc-705.
I don't know whether such properties of 50 cSt silicone oil can be a good fit for diffusion pump oil.
Regarding the "break down" temperature of silicone oil, I am not sure. But I hope that it's boiling point at 0.5 mmHg would lower than it's cracking temperature, so that I can use it as diffusion pump oil.
I know that highly refraction mineral oil (those clear transparent vacuum pump oil) can be used as diffusion pump oil, but they can be oxidized. (Even though a bit cheaper than silicone oil PDMS, and dc-704)