Need advice on MERV filter rating for older electric furnace.

I'm a first-time homeowner with very basic/general knowledge of HVAC systems. The house came with the original 1983 GE electric furnace (apparently actually made by TRANE) with model number BWV030P100D0. House is 1468 Sq. Ft. with about 12 vents (count might be off).

The previous owners left 20x20x2 MERV 8 filters behind (the filter gap is closer to being around 2.5" high leaving an annoying space between the return and the filter, but that's a different topic...), but I'm wondering if MERV 8 will be too hard on the blower fan and/or reduce the amount of air flow moving through the system?

So, if anyone can tell me the max recommended MERV rating and also how often I should be replacing the filters (like monthly instead of typical 3 months), that would be most helpful. Oh, we did have a duct cleaning company come in and clean out the whole system a week before we moved in.

Thanks.

submitted by /u/vader2000
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