Here is what the MSDS says. You shouldn't still be tasting butane if your oil is properly purged, but with a TWA of 800 ppm for 8 hours, I predict you are safe.

Contact with rapidly expanding gases can cause frostbite.

Target organs : May cause damage to the following organs: central nervous system (CNS).

N-Butane 106-97-8 100 ACGIH TLV (United States, 2/2010).
TWA: 1000 ppm 8 hour(s).
NIOSH REL (United States, 6/2009).
TWA: 1900 mg/m³ 10 hour(s).
TWA: 800 ppm 10 hour(s).
OSHA PEL 1989 (United States, 3/1989).
TWA: 1900 mg/m³ 8 hour(s).
TWA: 800 ppm 8 hour(s).

Section 11. Toxicological information

Carcinogenic effects No known significant effects or critical hazards.
Mutagenic effects No known significant effects or critical hazards.
Reproduction toxicity No known significant effects or critical hazards.
No specific information is available in our datab