I am using 20w50 which is an API of SE in my '80 350 LeSabre. I don't particular want to be using that high a weight of oil in the winter really. And rather the 10/40 recommended in my owner's manual. However, SE classifications of oil are only available in 20w50 form these days where I am from (UK) without paying stupid prices for import stuff. I mainly use this oil for the proposed zinc content that is apparently required. And would gladly change for the above reasons. I have found a 30 weight that I can get bulk in cheap, but is API of SD! And of course- zinc is not listed, of course. In forum member's opinions, do they stick with API standards, or perhaps use makes of oil with guarenteed ZDDP/Zinc content regardless of API standards? As far as I'm aware, The newer 'editions' of oil standards normally contain less of the material an older motor might need. According to what I can find, the classification after G is where zinc was reduced. So.. I could use a normal 10w40 F or G? Thoughts?