Don't bother wasting your time trying to slow down a clutch motor (been there, done that!), there is a steep learning curve trying to control one for sewing leather. You got those machines at a bargain price so do yourself a favour and buy a servo motor, you won't regret it (you may need a speed reducer too, but try the servo first). There's been lots written about oils, to a point it's not that critical as long as it's a light oil but do not use car engine oil in the long run it can gum things up. The best way for a newbie is to buy Lillywhite Sewing Machine oil if you can.
That 155 is a real workhorse.