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I have a 94 Mystic.
When changing the oil and filter, I don’t remove the lines to the filter cover.
There’s enough room to remove the 3 bolts and remove the cover and it hangs out of the way. Yes, there is oil in the cooler that does not get changed, but it’s a relatively small amount.
And I use the steel shim on the o-ring. Gives a larger landing.

Seth wrote: ↑Fri Oct 14, 2022 11:49 pm I have a 94 Mystic.
When changing the oil and filter, I don’t remove the lines to the filter cover.
There’s enough room to remove the 3 bolts and remove the cover and it hangs out of the way. Yes, there is oil in the cooler that does not get changed, but it’s a relatively small amount.

That's exactly what I do too. The oil in the cooler actually does get changed; it drips out of the hanging cover while you change the filter. Since there is no restriction in there to prevent the cooler from draining, gravity does the job. Most of the oil in the cooler also drains back into the sump on its own when parked for a while. The cooler and its hoses only hold about .25L of oil anyway so it doesn't make much difference on the dipstick.
Some older models had a totally useless thermostat in the cover; those needed a bolt inserted in them to drain the cooler.

Seth wrote: ↑Fri Oct 14, 2022 11:49 pm And I use the steel shim on the o-ring. Gives a larger landing.

+1. Me too! Always a metal shim between the canister and the white o-ring, and no gasket on the cover.