The ST170 engine has a MODINE oil cooler on the back, sanwiched between the filter and the block that uses the engine's coolant to keep the oil at a relatively constant level, I'd say that would suffice mate although you could go one better and get a 13-row mocal jobbie but it may be too much when combined with the one you have already!

Great BHP figure on the R/R by the way too mate well done!