Mobil 1 15W50? I know it isn't high mileage, but wouldn't it be the thickest on the market?
M1 15W50 is actually one of the thinnest 50 weights.
Viscosity @ 100ºC, cSt (ASTM D445) 18
GTX High Mileage 20W50 is 20.3
I want in on this p1ssing competition:
Penrite HPR 50 (40-70 mineral oil, no W rating) with a 100 C Viscosity of 30.5 cSt, and 1450 ppm Zinc.
OP, to answer your question, look at the oil data sheets for actual viscosity, but most Castrol oils tend to be on the thicker side. Also nothing wrong with a 10W30 HDEO or a 10W40 PCMO. Both would be thicker than a regulat 10W30 PCMO.
That semi-synthetic MaxLife 10W40 looks good and would be worth a try, so would a low restriction oil filter.
K&N filter may help more. It's a filter known for high flow rates. The Ultra has high filtering rates.
The Ultra will definitely flow better than the K&N. Not that any of this matters. The Ultra does both better because of the microglass depth media compared to the cheaper cellulose media in the K&N.
Also the K&N will be about 85% efficient at 20 microns while the Fram Ultra is around 99.9% @20 microns
You may find this PQIA HM oil test from a few years ago useful
The defy is now regular zinc, and no longer high zinc.
The GTX is the thickest 5W30 HM tested by PQIA. The GTX 10W30 HM would be my pick for a Dino oil, if 10W is OK for your cold starts. If you want a full synthetic, the M1 HM oils are also on the thick side. I very much like M1 10W30 HM with a HTHS of 3.5 and it's Euro A3/B3 rated. The M1 5W30 HM has a HTHS of 3.3 and so thicker than the regular M1 5W30 with a HTHS of 3.1 cP.
For semi-synthetic, hard to go past red bottle Valvoline MaxLife, but I might be inclined to step that up to MaxLife 10W40.
Go with whichever you can get cheaper, both are excellent oils. People here get way in over their heads in the thickest/thinnest debate. Your engine doesn't care which one you run, as long as it's the correct viscosity cited in the manual!
At the end of the day, oil's job is to lubricate, clean, and help keep the engine cool. As long as it's doing that, you've got nothing big to worry about.
When I had a noisy engine I poured Mobil 15W-50 into it (or maybe it waa 5W-50... can't recall). The Honda service advisor looked at me like I was nuts, but I told him, "It's 90 degrees outside. It will be fine."
Try the really thick oil.
I think you're going in the wrong direction then by wanting a thicker oil... If the issue only exists at startup then you want THINNER so it flows quicker to the tensioners and creates the hydraulic pressure needed to keep the chain from slapping. This was a common issue on the 4.0 fords and usually oil alone won't cure it... Try a 5W20 and use the Lubrimoly MoS2 for an OCI and see what you get.