Sum of cells in a column

You can’t get directly at those SUM totals to the best of my knowledge… 

You mentioned not want to use a rollup, but have you considered that rollups can now filter the records they summarize (this is a fairly recent addition, in the last 6 months I think). Without knowing exactly how you calculate when the oil change is due I can’t precisely advise, but I’d envision a system that rolls up your trip counter until it reaches X number in Tab 2, which then triggers an automation that sends you an email and checkmarks a box on the records in Tab 1. The rollup has a filter that only displays unchecked records, so then when you log the next trip it starts adding from 0 again. 

That’s also assuming you get it changed at due and doesn’t account for actually logging the oil change, but the principal still applies I think.

When you added at the end cause it has to be “manually updated” that jumps out that you should create an automation for that.

It sounds like you have a table of trips which contains vehicles.

Vehicles should be a linked record to the Vehicles table which acts as your rollup.  You would have a rollup in Vehicles that sums the total miles.  I would add an oil changed single select field (or checkbox) in Trips.  Then when you mark in Vehicles table that an oil change has been performed, you change the value in the trips table for all of the trips for that vehicle since the last change.  Then your Rollup in Vehicles for miles driven can only sum where Oil Changed is no (from the Trips) table. 

This would fully automate that process.

This works flawlessly and exactly as I imagined. Thank you all for your replies.

You can take a look at how it might look through an interface

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Thanks for inspiration 👍

Looks like I didn’t share the base.  you can interact with the interface.