The mobil will have some zinc, but if it complies with the latest oil standards then it doesn't have as much as it should.

15w40 is definitely too thick for your climate. I had trouble with 15w40 being "clacky" on cold start I 10-15 degree weather. I've since switched to mixing Rotella 15w40 and 5w40 50/50. That makes a nice median 10w40 weight (which Rotella doesn't make, for some reason) and has cleared up my issues.

I would recommend switching to a larger filter. Gives more filtration area and a little more oil capacity. The Mobile 1 M1-301 is my first pick. Second to that is the Wix 51515XP which is their synthetic filter (not specific to synth oil; synth filter material) and third is the Wix 51515 which is still a good filter and is a lot cheaper.

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You will not be able to tighten up the inboard valve cover bolts as they are under the intake manifold, FYI.

 

Chiming into say from my experience, if using 10w-?? and do not notice a change in starter speed during cold starts there would be little reason to change to a lower viscosity.

Diehard 20w-50 user and when living in cold climes would usually just top off the battery charge and go.
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Judging from the pattern, something sprays oil. Besides the rear main seal, check blanks plugs on the front cover.

A 2004 Disco does not have oil cooler, so where oil cooler lines were supposed to be, there are blank plugs. If a plug is loose, it will leak oil and that leaked oil will blow back hard all around the truck. Watch the video on the AB page below and pay attention to the information at 1:43.

http://www.roverparts.com/resources/...-engine-cover/

When I got my truck, a visual inspection revealed a pretty bad oil leak which turned out to be a loose blank plug. Once that was fixed, the Landy became... Are you ready for it? Dry. I kid you not, my neighbors Japanese cars leave more marks than my beast.

As for oil, I run Castrol synthetic 10W-40 with ~4oz Mystery Oil. The truck is fairly quiet and I am happy with the performance.