btm
Our saying is “ your boat, your choice”, but I would take with caution any recommendations to use an oil w/o that recommender knowing specifically the product you have in your hand for your engine to injest.
Look at the manuals, you want an API service category of CF, CG-4, or CI-4.
CF is obsolete, tho some mfgrs might still list it if the product meets it.
IIIRC, T-5, T-6 do not meet those APIs.
To my knowledge, Kubota does not offer a synthetic or blend -which says something to me. Check Messics.com for Kubota fluids information.
Lastly, you could email Westerbeke to see if it offers a synthetic or blend.
The common wisdom that I’ve read most is to stick to T (probably NLA), T-1, or T-4 depending on the grade you want to run for your climate.
you may find these useful:
Oil Categories
good luck and let us know if you find out any specific info.