I have a 1983 Stihl 026 that I had the dealer put an 026 Pro adjustable oiler in. I also have an MS-362 for big stuff.
As everyone suggests, I clean out the bar with a putty knife and compressed air, take off the little metal plate behind the bar and blast that off, too.
I tried "big box store" bar oil and found that it worked very poorly when cutting oak. The chain would "smoke" - NOT good!
I now use Stihl or Husqvarna oil. The gys at work really like the oil from Bailey's Supply.
The biggest thing is probably chips and debris getting into the oil tank, when it is refilled in the "woods". I let the saw cool off a bit, and then pour just a tiny bit of saw mix over the oil plug to get the junk off there before I pop the cap. That way, the oil stays "clean".
For really cold weather cutting, Stihl also makes a winter grade bar and chain oil.
If the bar comes into contact wth the "dirt", that could also partially clog the bar groove, cutting down on the oil.
SC