See, to me oil and soap don't mix therefore will cause issues getting a decent lather from your soap. whenever I've used oil I never get a nice thick lather, which in itself causes razor burn for me.
I personally don't use oil, and have never seen the need for it, just a good quality soap should do.
I use Castrol Synthetic oil on my straight razors. I think the synthetic will make them cut faster and last longer than ordinary oils. I am hoping to shave in half the time, once the synthetic fully permeates the pores of my old Sheffield straight razors.
I used 100% unscented glycirin soap and make a great lather. Towards the end of my shave I decided to put a little baby oil on the end of my razor and I was amazed on how smoothly it was cutting compared to before.
However I wasn't to sure if it wasn't just in my mind or not.