oil pressure problems on air dryer

We are having problems with an air dryer that has a 50hp coplematic 8R/8D compressor running it and were looking for confirmation of our ideas or any new thoughts on what might be the problem

We have had a number of rebuilt compressors on this unit and have been having oil pressure problems within a very short time(1 hour) to the extent that the compressor is dropping out on the oil failure control. Upon stripdown of these compressors all rods and main bearings are showing signs of lubrication problems and wear
The unit runs on R22 with 3GS suniso oil and is a fully flooded vessel, it runs 24/7 and is in an unloaded state for a significant amount of time either 25% or 50%

Some pressures and temperatures are as follows
When load is 50%:-
Discharge pressure - 1300kpa
Discharge tempreture (12-18" from valve)- 81 deg C
Suction pressure (@ compressor) - 350kpa
Suction temperature - 9-12 deg C
Evaporator pressure 410-420kpa
Sump tempreture 57-58deg C (probe positioned on outside of sump directly above the oil filter plug)
Oil temperature from oil seperator 65deg C (temp taken on 1\4 return pipe 8" from compressor)
Oil pressure 150 kpa

When load is 75%:-
Discharge pressure -1300kpa
Discharge temperature (12-18" from valve)- 85deg C
Suction pressure - 350kpa
Suction temperature - 14deg C
Sump tempreture 57-58deg C (probe positioned on outside of sump directly above the oil filter plug)
Oil temperature from oil seperator 65deg C (temp taken on 1\4 return pipe 8" from compressor)

given the above we would like to know the following
1/ Given that the probe is strapped to the sump and not actually in the oil would it be safe to assume that the oil temperature was significantly above the probes reading? (as much as 20 deg C??)

2/ Assuming that the oil temperature is elevated to approx 80 deg C would it be expected that the oil would lose its viscosity to the degree that wear would begin to occur and/or oil pressures drop

3/ Are these pressures and temperatures within an acceptable running regime and if so why would wear begin so early or oil pressure die so quickly (bearing in mind that all tolerances were correct at the time of assembly of the compressor)

4/ can you suggest any fixes for this problem (do not hesitate to ask for more info if you think you can help)

sorry for the long winded explanation but i think you need to know as much as possible before you can see what our problem is

Edge, the sump temp will give an aproximate oil temperature, within say 5C the oil would not be 20c higher than the sump. Again you discharge temp seems very high, especially for a flooded system, I would say 70c would be the max normally seen here. Condensing temperature seems low, coupled with a high superheat this system may be short of refrigerant. Another possibility is you are running on hot gas bypass, this is commonly used on air driers to obtain a constant 5c evaporation, again you evaporation seems slightly high. A drier with the facility to hot gas bypass on low evaporation, will do exactly that when short of refrigerant. Another secondary effect is when using a flooded vessel the oil will not return correctly from the evaportor, when your refrigerant level is low, as the oil/refrigerant mixture for rectificationn is taken off at the normal run level on the vessel.
I hope the above helps a little.
Regards. Andy.