Brad, Doc's right, the dampener oil only affects how quickly the pots rise. So, heavier oil won't affect cruising, it'll only affect acceleration.
I've no idea what grade Air Tool oil is, I just used it because 1018cc uses it! And he uses it - I think - primarily because it's fairly light, it's something you use elsewhere and therefore have lying round the shed, and it's cheap! It could be right, could easily be very wrong! For your engine, and for mine.
My GUESS from your numbers is that, currently, the oil's having much more impact on your dashpot levels than your springs. Constant cruise with the H6's is too rich, yet acceleration's lean? That would make me think that the dashpot's rising too far (which is the spring rate), but it's probably rising a bit too slowly..? Then again, if you change springs, the dashpot oil may need to change again. I remember DrMini saying that he (and GR?) discovered that one of his cars needed dashpot oil only in winter - the rest of the time, the oil slowed things down too much.
What I'm confident about is that changing your springs WILL improve your cruise mix. Currently, you're running rich. So, you want to run leaner under constant load. You want to run on a thinner part of the needle for a given load, a lower station number. You want the dashpot to stay lower. Lower compared to where it sits at WOT at peak torque. Which means you want a heavier spring. Which, from a Red, is a Yellow.
Don't forget, the moment you change the springs, all the readings you've taken with the LM-2 will be different. Relatively, the needles will be richer and leaner as they currently are, but my expectation is that the M's will go from being lean to being a bit rich, and the H6's would go from close to being VERY rich. Which ties to WinSU advising you to use those super-lean needles you specified back on the other page.
I hope those MG boys open up again for you on Monday! Though that's still too far away! And I'm glad you tightened up your spark plug gaps.
