Pro's and con's
WD40, kept my chain the cleanest it's ever been but destroyed it twice as quick as 'normal' oil. But ive heard other manage quite well on WD. But then there is the environmental cost to consider as well. Hardly ideal using aerosols twice a week.
Engine oil, (was) super cheap, quick, easy, but left my chain wet and very dirty, lots of oil droplets everywhere, chainstays, rims, hubs, everywhere. Very dirty
Chainsaw oil. Quite cheap, less 'fling' than engine oil but still 'flings' a bit. Chain gets dirty.
Bike specific chain oils, tried various (barring the expensive ones)...not much between them.
Ultimately, i used to monitor chain mileages quite anally, barring WD, most chains wore out about the same time, whatever method i used.
All these are just my experiences, others may differ.