The "400" oil Tamiya is referring to is rated in 'cSt' or centistokes. It is a measurement of viscosity instead of weight.. Unfortunately cSt does not directly translate to a weight equivalent.. In other words, 400cST oil is not equal to 40WT oil. It is actually just a tad over 30WT.
Here is a pretty good chart posted by XRAY that gives you a more accurate comparison:
http://www.rceasy.com/2008/01/10/xra...version-table/
Hope this helps.