I've never used anything other than the factory recommended 10 wt. oil. One of the issues with these forks is they're not real responsive, especially to the little bumps, and I feel a heavier oil would only make that worse. This is one of the main benefits of doing the Minton Mods, they improve the response. You do want to use a little more than the stock amount (just under 6 ounces). This will greatly reduce diving during braking and the forks just plain work better with more oil in them. On forks I've had all apart, totally cleaned, and are completely dry, I put 7 ounces in. A drain and re-fill gets about 6.5 (because you never get it all out). Or as mentioned above, you can set the level by measuring, 6" from the tops of the tubes, springs removed and forks fully compressed.