This is a valve adjust shim I put in upside down many races and street miles ago. The etched size markings are still visible. This illustrates why it is critical on these Triumphs to use the correct oil. These are demanding engines. The oil lubricates the transmission, the engine, and the clutch. It is also used for cooling and the head has flat tappets.
The essential ingredients are the additives to make all of this work and it is a good idea to verify they are there. This product data sheet from Mobil illustrates what to look for. The JASO certification, the statement the additive package is for a bike, and the correct phosphorus and zinc content.
This bike has run on Mobil 1 for its entire life. The reasons are I can verify the additives, zinc, and phosphorous are there, Triumph recommends it, almost always it is on sale somewhere around here and I can get it for a good price, and I see no oil related problems during the tear downs. Other oils might be as good or better. South Bay Triumph uses Torco in their turbo bike with success and some folks I know use Silkolene.