Oils Experience

Run it on semi synthetic 10/40 rather than a fully synthetic oil.

 

Most semi-synthetic blends have very little synthetic oil them. Some have only around 5%. It could even be less since there are no regulations to control the amount required to be labelled and sold as a semi-synthetic.

 

Triumph tend to recommend Castrol, but Amsoil is good stuff with a solid reputation.

 

A fully synthetic oil for a Trident 660 is not necessary, it is more for the performance machines such as the Speed Triple 1200 RR and RS, as well as the Street Triple 765RS, using a fully synthetic oil in a 660 can give clutch issues such as sticking plates. Triumph recommend a semi synthetic oil for the 660, as learningtofly says Triumph recommend Castrol, any good quality specific motorcycle semi synthetic oil will be fine. I run my STRS on fully synthetic oil, due to doing some track days and spirited road riding. You don't mention if you bought your machine new or second hand, if purchased new or within warranty second hand, then you will need to have your machine serviced at a Triumph or independent dealer, otherwise any warranty claim for a home service invalidates the Triumph warranty. Triumph confirmed this via an e mail to me some while back. Hope this helps.

 

Just look for the correct grade and spec. Other than that it doesn't really matter what the oil makeup is IMO although I doubt you'll find a mineral oil that meets the specs required by modern bikes as emissions also play a part. I just look for a SS oil of the correct grade/spec at the best price rather than sticking with any particular brand. FS just costs more for no real benefit IMO. Just me 2p.