Going to need more info on this one. 58 degrees on its own is not a problem for the oil but something must be happeing with your system. On our turbines the oil cooler outlet temp (ISO 32 oil) is 50-52 C. 58 C would concern us but only becasuse it is higher thatn normal and as a consequence the return oil temp from the bearings and load gear would likely be higher. Our hottest return temp is around 86 C and we don't want to be much warmer than that as the oil starts oxidizing.

Is there a temp increase in your oil outlet temp from the bearings or in your reservoir?

Also, what was the temp before (i.e. what is normal)?

Presumably you have started looking at what is different. Do you have anything to share?

58 degree C at cooler outlet, what is bearing oil outlet/ babbit temperature? Higher temp may result in reduced viscosity , low load bearing ability, metal to metal contact as well as reduced natural frequency of the rotor. It may also lead to instabilities.
Irshad

Normal cooler outlet temperature is 40ºC, we are having problems at heater exchange because pipes are blocked for a kind of mollusk we have in our river.
It is not a normal operation status, bearing temperature is near 70ºC (average).
Inlet oil temperature is over 65ºC.

That is a big jump, I was thinking more of a smaller increase over a period of time. Akhtar's right. The biggest concern is viscosity in the load zone. The oil temp is the only variable to your lubricant film thickness. I would be concerned that you may not see the increase in temp in the bearing rtd's/thermocouples before you wipe the bearings.

from what i had so far for turbine/compressor, normal vendor practise: 46 deg C for ISO VG 32, 52 degC for iso vg 46 oil, as for vg 68, if have not encountered yet but 58 seems close to its ideal operating temperature. you can always refer to viscosity vs. temperature chart for all viscosity grade.

if you see no abnormality after running more than 1 day at current temperature, then it should be ok. one of the thing that i have experienced with if you have bad bearing desing is morton effect that it may happen at low or higher band of your ideal operting temperature of oi supply..

you may want to run both coolers if you still want to reduce the supply temperature..

slight reduction in radial vibration could be the thing you may see when running with higher oil viscosity