Didn't know the recommended interval change is 5k for EO..a bit too long for our weather imho

Nothing wrong, just my personal preference.

Mineral - good base oil good additive - jimat
Semi - really need to study the specific oil blend to trust it. For normal humans like us better just skip. Can go LYN oil thread and check out.
Fully - confidence

Semi syn contain more than 50% mineral, at least 70% or more, IINM.
So if the mineral oil breaks down, it breaks down as a whole. The remaining synthetic blend will not hold for long.

I think most people believe going from mineral, to semi, to full, is a 'straight line' thing. Meaning, if cannot afford fully so get something in between and have a good feeling. Or, in other words, mineral 3k , semi 5k, then fully is 7k? IMO, it doesn't work like that. Synthetic oil is extracted from petroleum or mineral gas (GTL) then, engineered for enhancement... then, mix back to mineral oil will make it better?

I guess it should be more like, mineral 3k, semi 4~6k, fully 7k. Quality of semi oil depends on educated guess and luck.

Not to say all semi is bad. Just emphasize the information takes more energy to digest. Maybe you're lucky that you picked a good semi oil. No problem using it as long as it works for you.