[Ecky]

Not 0w16 but related: I'm considering running 0w20 in my engine after I run through my stock of 0w30, and could use some opinion.

The K24A2 in my car calls for 5w30 and was never later spec'd down, while most other Honda engines from the period were. For example the K24A4 in the Accord was originally spec'd for 5w20 and later given "0w20 also ok". That engine is extremely similar, but has lower compression (9.7:1 vs 10.5:1), a lower redline (6800rpm vs 7100rpm), and less horsepower (160 vs 200) which is mostly from camshafts and smaller intake and exhaust diameter. Aside from different camshafts, pistons and rods, they're basically the same. Bearings even have the same part number.

Later revisions of the K series all call for 0w20, even ones with the same horsepower and redline. But again, Honda did not back-spec this particular engine, when they did that for most others. This is the highest factory torque and horsepower of any K series.

I live in a cool climate. Without a grille block, I have a hard time getting it up to temperature any month of the year, but especially in winter. I have no way to monitor oil temps but I expect they're not high. I'm planning on adding the coolant/oil heat exchanger from another K series soon, mainly to help the oil to get warm. I have also reduced the rev limiter from 7200rpm down to 7000rpm.

On the other hand, I have an under-driven oil pump (which probably still has pressure limited by the relief spring). I run much more advanced ignition timing (5-7° at WOT), meaning there's more force being applied to rods and bearings. I also do drive spiritedly in short bursts - it's not particularly rare for this engine to bounce off of my rev limiter.

Thoughts? 0w20 probably fine? Even thinner? What would you do?

Edit: Under what operating conditions will lower viscosity oil most likely become a problem? Is it low RPM high cylinder pressure, or high RPM operation?