Just remember, if you're letting your car warm up by ideling for 20 minutes, that means only the engine will be warmed up. Gearbox/diff need time to warm up as well.
Another thing to consider re your oil consumption is you PCV.
20 mins of idling could use the same fuel as 10 mins of normal traffic driving. Do that 10 times a week and that's like 40-50kms off the range of a tank.
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