I've been using 15W-40 mineral in my revo, sometimes 10W-40 semi, API C rated of course.

After reading comments on this forum, in which can be summarised as: "follow what's in the manual", I was curious & decided to change to full synthetic API CI-4 5W-30, Total Quartz Diesel 9000 to be exact. (The manual recommends 5W-30 or 10W-30, API CF minimum)

Yes, the engine feels light & rev up easily as expected, smooth sounding & quiter also. But somehow as it revs, I feel like the engine is missing that familiar 'torque' that was prominent when using 40 weight oil. The quick light rev is there, but it's missing that 'grunt', especially noticeable when climbing hilly area (I mainly drive off road, hilly & muddy area).

When it was time for an oil change, I purposely drive it on the hilly area that I normally drove in, reach home & immediately drain the oil, hot hot oil. The consistency of the oil is like water, like your tap water, flowing like kopi o.., whereas previously a 40 weight oil at the same situation will still have it's 'oil' consistency. And my guts telling me that this isn't right, this thin consistency can't possibly protect my engine better than 40 weight oil.

Btw, that's just my experience, I think it's related to the severity of application. For my type of application, I think heavier 40 weight oil is more suitable. Maybe for city strolling, light duty application, lighter 30 weight oil is more suitable.

Cheers