We have a gearbox that uses EP5 ISO VG 220
gearoil. We are having trouble with one gearbox foaming. It is outside ambient temp at night is between 30 and 45 deg F it only foams at night.
What is a good anti fom additive.

What about 10-15 ppm Silicon oil?

Tim B,
Mr Hughes is correct Silicon oil is normally used as an anti foaming additave, at the levels he has suggested. However too much of a good thing is worse than not enough. If there is only 1 gbx that foams then chances are that there is already too much silicon in the oil ie perhaps a recent overhaul where the mechanics used too much RTV silicone sealant to join the casing and that is leaching into the oil.
The fact that is is only at night is interesting. Higher production (working harder)perhaps? I have not heard of anything other that a cooler oil retaining the air entrainment for a longer time. However that should apply to all of the gbx's?
regards......