I'm watching the series again (again), and noticed some inconsistencies with the appearance of the scorched engine oil, so I figured I'd throw it out there to see if anyone can put an observation on this that explains what I'm missing.
So here's what I believe to be true:
1. Scorched engine oil (or something with the same properties as scorched engine oil) is used to access the Black Lodge. I recall reading somewhere - although the 'where' escapes me - that it is also present in the stone-edged pool within the circle of sycamore trees in Glastonberry Grove.
2. We know the smell is associated with BOB. It is present in the train car, as evidenced by Ronette's violent reaction to it when questioned; further evidence of this comes from FWWM when Leland, confrontedby Mike/Gerard, revs his engine to cover the same smell.
3. Jacoby first notices the smell at the gazebo, confirming BOB as his assailant and the cause of his heart attack. The smell does not appear to linger; many of BOB's attacks on Laura took place in her own bedroom, and the Maddy murder took place in the Palmer home, but no record of the smell of burnt oil there is acknowledged. There is also no mention of it in connection with Theresa Banks' death, so it is probably safe to assume the smell comes directly from BOB's presence.
4. There is nothing to suggest that either of the two people who react to the smell (Ronette, Jacoby) have any capacity to see visions. Jacoby never gives any indication that he has ever seen BOB, and Ronette does not realize that BOB is hosted by Leland. In fact, no-one sees them both, leading me to believe that the 'mirror shot' that was used in the show to show BOB's inhabiting presence was intended to be from the perspective of the host (i.e. Leland, Cooper). Laura does, fleetingly, see her father during Bob's final attack in FWWM but at this point she is already all but certain that she knows BOB's real identity.
Which leads me to this: Why did Jacoby smell scorched engine oil at the hospital? He reveals this under hypnosis, and it is this 'evidence' (and ONLY this evidence, by the way!) that leads to Leland's arrest for the murder of Jacques Renault. I understand he would have smelled it at the gazebo, but the murder at the hospital was not a BOB act, it was a Leland act. I reach this conclusion based on three known facts: 1. Leland confesses to the crime. Leland had no recollection of his 'BOB' crimes. 2. Jacoby recognized Leland - "I know that man". If he had seen BOB, he wouldn't have known of him, and Leland wouldn't have been fingered. 3. Most tellingly, no letter was placed under Renault's ring finger.
Furthermore, if Leland exudes the 'essence of scorched oil' (hey, perhaps they could sell it at the perfume counter!) when he is overcome by evil - or something - why doesn't Cooper smell it when Leland is about to wrap a golf club around his head?
Thoughts and explanations needed, please, and if you've really read this far, thanks for playing!