[abstruse1]

The manual calls for 15W-50.  There's a type of oil I'd like to use, but it's made in 10W-40 and 20W-50, not the specified oil.

Does anyone have any idea of whether either of these two viscosities would work fine in the GT?

[Kevin Mahoney]

Will work as long as it is at least semi-synthetic and a good motorcycle oil

[suitcasejefferson]

Won't get into brands, but I would definitely use 20w50. It has more load bearing ability than the thinner oil does. Long stroke engines need all the load bearing ability they can get. High revving short stroke engines can get by with less.

[Roeland]

Why not play it safe and go full synthetic?

[abstruse1]

Amsoil 20W-50 motorcycle oil it is, at least until next winter.

It gets HOT in Texas.