Which Oil for Kanzaki KMZP-1?

[vw1961]

Hello!

I have a Yanmar 3GM30F (European Model) with a Kanzaki KM2P-1 gearbox.

I own it since about 2 years and it never gave me any problems, still i think it is time for an oil change, but there is some conflicting information regarding which oil to use:

1) If I check the dipstick, there is no oil missing and the oil looks really clean, but it has a red-ish colour, so i suppose the previous owner might have put ATF oil (which is incorrect i suppose)

2) The manual says same oil as engine (for which i use 15W40)

3) The plate directly on the gearbox says "SAE 20/30 HD" and from all three options i trust that one most.

I think i will use SAE30HD engine oil. Any other opinions? In case there is ATF oil inside now, should i flush it a few times or isnt it a problem if a little bit of ATF stays inside?

Thanks for any opinion or experience