We usually use the HM46 or HM68 in normal conditions and works good.If that problem is only lowering the boom,thats due to a malfunction of the security valve located in the hidraulic cylinder.
When you lift up,the circuit is free,but when you try to lower,you need a "counter-pressure" in the valve to open the lower the circuit.
Anyway,if you doubt about the oil grade and condition,change it.
Best regards
R.Feitor