Small Engines (Lawn Mowers, etc.): front dif. of a 1975 rotor tillar, motor model, gear lube

Question
rotor tillar: model: 75 sheffield 12-50.15
serial: 11851
Briggs motor: model 130292-type:0141-08 SER: T4110407
problem: after 20 minutes of tilling, the machine slows to a crawl and eventually quits...much like as if there was not enough oil in the motor. Motor seems fine.  Front dif...when I open the plug, it often spits oil out.  Tried draining some??? Do you think that maybe the dif oil I put in isn't thick enough...what type would you recommend?

Answer
80/90 weight gear lube.  It sounds like the gearbox is failing.  Remove the tines and disassemble the gearbox.  Inspect for worn or broken parts.  Replace as needed.  Inspect the tiller and see if you can find a model that begins with GIL.  If so it was manufacturer by Gilson.  That is owned by Toro now.  They may still have parts for this tiller.  Give them a call or visit the website.