viscous fan problems

During our trip round Aust last year, our 80 series 1hz diesel cruiser began to overheat
(only when "working" in high outside temp and aircon going. We think the clutch fan isn't kicking in, and we were told that you can actually add more viscous oil, which we tried recently. I found that an aftermarket fan is about $150.oo, so perhaps it's just worth changing. What do you think?