After pondering viscosity and HTHS and my vehicles, I had a couple of questions that I thought I would ask on here. My cars spec 5W-20, 5W-30 and 10W-30, and I've been running 5W-30 in both for the majority of the time I've owned them. They're both port injected, naturally aspirated, and seem relatively easy on oil. Anyways, I started looking at different 5W-30 oils and got to wondering something. One of the oils I looked at was Motul 8100 X-Clean EFE 5W-30. This is an API certified oil, so I picked out another API certified oil to compare it to. This version of Motul would be perfectly acceptable in both of my vehicles because, as the owner's manual states, any API certified 5W-30 is fine. Here's the stats on Motul 8100 X-Clean EFE, and a few highlighted below: Visc. @ 40C: 70.1 Visc. @ 100C: 12.1 HTHS: 3.5 Now, as I stated, this oil would be perfectly fine witth regards to the warranty, as it meets the stated requirements. However, lets compare it to an oil that would be outside the requirements of my warranty. Mobil 1 0W-40, a well regarded oil on here, would be an oil that could technically void my warranty if I ran it as it's not a recommended viscosity. However, here's the stats for this oil. Compare it to the Motul 8100 5W-30. Visc. @ 40C: 70.8 Visc. @ 100C: 12.1 HTHS: 3.6 Now, I'm sure I'm missing something obvious and I know Visc. @ 40/100 isn't everything, so please excuse my ignorance on the subject, but why would the Motul be a 5W-30 and the M1 be a 0W-40? I'm also assuming that, since the obvious thick 5W-30 would be fine in my engine (per the OM), the M1 0W-40 would also be perfectly fine (though outside the recommended viscosity)? I thought I remembered reading that a 5W-30 couldn't have an HTHS above 3.5, but I could be wrong. Is the .1 mPa.s make that much difference? Thanks! Note: Please leave brand out of the discussion as much as possible. I know that any 5W-30 is fine (IBTQSUD5W-30BestValueAtWalmart$19.50), I just had some questions about how two similar oils could have a different viscosity and one be within my warranty and one outside.