I thought this was common knowledge, & I've read several warnings related to this topic (GL-4 vs. GL-5).
I would start, however, with most oil manufacturers recommended using the type of manual transmission oil recommended for a particular vehicle -
My 2017 Mitsubishi Mirage owner's manual recommends -
Mitsubishi Motors
Genuine NEW
MULTI GEAR OIL
ECO API Classification GL-4
Viscosity range SAE 75W-80
Since GL-4 Gear Oil 75W-80 exists, I would have thought the discussion ends there. Redline MTL is an example used by most on the forum.
To address your lack of concern - This is the most common thing I have read on the topic-
"The main difference between GL-4 and GL-5 gear oils is the amount of EP additives. Sulphur/Phosphorus containing products are used as EP-additive. This additive has the purpose to prevent the occurrence of micro-welds on the gear flanks at the local high temperatures which prevail in EP circumstances (temperatures well in excess of 800℃!) GL-5 has roughly twice the amount of EP additives compared to GL-4, which is why it is often used in high-pressure circumstances such as in a front axle and rear axle differential.
Sulphur/Phosphorus additives however have an unfavourable property: they can react aggressively towards bronze and copper. This can be disastrous for the synchromesh rings of a gearbox. Therefore it is not recommended to use GL-5 in a gearbox unless the manufacturer allows this."
taken from
https://www.rymax-lubricants.com/upd...gl-4-and-gl-5/