I have an old fiat AD 7 dozer and when the hydraulic oil get hot the hydraulics stop working is there an additive that you can add into the oil with out damaging the system to stop this problem or are there any suggestions how to solve this problem .
You may be describing a symptom of a problem shumba. It depends on what temperature you're describing as "hot". When hyd oil in a system gets 150 - 180 degrees F (65 - 80 C) it will shear inside the pump and cause all kind of operation problems, not to mention the component damage that's taking place. It's very likely that something is wrong (the problem) that is causing the oil to get hot (the symptom), such as worn pump/s, faulty relief valve/s, oil cooler clogged, and I'm not sure about a Fiat, but some machines use a cold oil bypass system that diverts oil around oil cooler until oil gets warmed up that could cause your problem. My point is, if the oil is getting hot, you need to look into why it's getting hot.
As far as I know the fiat AD7 didn't have a oil cooler and the relief valves have been checked so it must be a worn pump then .We operate our machine in an average outside temp of about 40 degrees c .Can you add any additives to the oil to maybe make it thicker so that it will work with the warmer temps
No additive that I'm aware of that will do what you're asking. Depending on what viscosity oil you have in system now, you could use a thicker viscosity oil. But I'm saying you need to check your oil temperature, if it's getting into the 65 - 80 C range, likely there is something wrong in the hyd system causing oil to get hot. Thicker oil won't remedy that problem, it will get hot as well.