What's the difference between oil guns & grease guns. Or, put another way, what's to stop me taking the grease out of my tecalamit pump grease gun and filling it with oil to oil the ways on my milling machine? Come to think of it how would I know if it was an oil or grease gun anyway?
Somewhere, carefully filed away (i.e. irretrievably lost), I have a copy of Lautard's "A Treatise on Oiling Machine Tools" which describes pretty well that (amongst other things). As far as I remember, it mostly consists of making a tip that matches the Myford oil nipples and improving the piston seal in the grease gun by adding a leather washer. A good quality grease gun is recommended to ensure that the pump cylinder is reasonably well-fitting.
I tried it with a cheapo Sealy gun and found it leaked too much from the pump area to be useful. Since then I've picked up a couple of Tecalemit guns but haven't tried converting them yet.
But the design does seem a bit odd : the pump is a displacement pump that traps a small container full of oil, cutting it off from the reservoir and using the lever pressure to force it into the spout. So it's sucking oil out of the reservoir.
Lautard's mod (I've found it now) removes the filler piston and seals the hole with a rubber cap. So surely you can't suck for long on the reservoir before it stops working ? Or does the pump action return some air to the reservoir on the release stroke ?
I did it with the grease gun from the toolkit of an old British car. It has less leakage than the Myford version. Up until 1970 or so, UK made small cars were popular here on Vancouver Island. Most of the grease guns were unused, as better ones were, and are readily available at affordable prices.