A B737 Captain was informed of an oil leak under the left engine by the Ramp Agent upon arrival. Investigation revealed that it was a significant fuel leak and that the aircraft would be grounded until repairs could be made.

Narrative

After parking and shutdown at gate; we were informed by ramp personnel of an 'oil' leak under left engine. Upon inspection we concurred; called Maintenance Control who arranged to have Contract Maintenance take a look. We felt that the leak was slow; but due to wind we could not get a good feel for the number of drops per minute. After Maintenance removed cowl and had us run the engine; we were quite surprised to hear that this was a huge leak of fuel; not oil. Followed up with Maintenance Control to make sure Contract Maintenance was correct in their diagnosis and was told that in fact they were and it was fairly serious due to location and severity. Aircraft was grounded and estimated time of repair was for the next day. We left ZZZ with this aircraft with oil quantity just below three gallons (limit 2.5). I knew that they should be doing a daily inspection at ZZZ1. While enroute the First Officer's radio Altimeter failed; requiring Maintenance (early alert). When we arrived Maintenance fixed the R.A. and we were told daily would be taken care of when we got back from ZZZ2. I asked that they throw some oil in as precaution. Just prior to start oil quantity showed close to four gallons; and I failed to look at the log book to verify since we were now at departure time as book was handed to me. It turns out; no oil was added. Had the mechanics either accomplished the daily or put the oil in; maybe the leak would have been discovered; although this is impossible to know. Also it is an education as to how a 'small' leak can fool you; along with trying to determine oil vs. fuel. Ramp agent deserves credit for being vigilant and bringing his concern forward.