405 g/box & o/drive oil advice

[GlennBurnage]

HI, I've had a good trawl through the forum and can't find the info I'm after, so can anyone help with the following question?
I know that the manual specifies sae30 oil for the linked g/box and o/drive on 405s. Does anyone know if this is still the best (compromise!) or is there a modern alternative?
It would appear that the 30 grade is only recommended due to the o/drive not liking anything thicker. Has anyone tried a more viscous grade that might be more suited to the g/box as I feel 30 is, in an ideal world, nowhere near up to the job
I feel that if these units were designed today, there would be a modern, possibly sythetic oil specified.
Any help gratefully received before I go out and buy some sea30 (or even try some 40!!)