I have used Stihl for 40 years & now have two. A little 170 for small jobs & camping & a larger 391 for use around the property. The 170 hasn't seen a lot of hard work & loses bar oil. Remember this oil is very, very "non-viscous" if there's such a word? "Runny or Thin" then!
Now, I'm definitely no mechanical genius but I find these machines easy to service & maintain. The larger saw is easier as it is not as finicky in size. So, pull the cowling off & give everything a good clean with petrol or similar (not degreaser) & tooth brush, check all hoses, wires etc. Replace air filter , fuel lines & fuel primer setup if fitted. You can buy these as a kit. Have a good look for wear on the chain drive sprocket (& bar tip roller if fitted) & check the groove width is standard right around the bar. A new chain would be the go too as the old one may not have been sharpened correctly & if you cut anything with a large diameter you'll struggle a bit by cutting in an arc.
Unless there's something major wrong they're a wonderfully simple machine to work on. (Being the age it is the carby may be a bit dicky)
Good Luck
Steve