If it tastes of oil it's probably an M110. Or an M90 that's been dipped in oil. Tricky business see....
Generally springs do fall into a range denominated by the number in metres per second so an M100 should give a muzzle velocity around 100m/s (or 328FPS). BUT - there is a difference between manufacturers as ably demonstrated in this chart: http://www.airsoftcanada.com/SpringChart.php (note that that is from Airsoft Canada and doesn't include many of the brands that are easily purchased here).
Muzzle velocity is also HIGHLY dependant on a number of other factors (Cylinder volume to barrel volume ratio, quality of airseal etc) so the spring strength should only ever be a rough guide. I have personally had issues with M90 springs resulting in guns with 350+fps (when combined with double seal cylinder head, high quality piston head, o-ring airseal nozzle and a decent hop rubber).