Please note - I am not a mechanic or expert in anything.
I did not realise your road was all dual-carriageway I would be very bored with that after a 5 or 10 mile blast I like roads not much wider than a narrow car.
I do not know the rounding of numbers in the calculator I have but on your ratios with 165/70/13 tyres it comes out with (if(?) I have input correctly and converted mph to kph correctly). ETA: the figures in the brackets are from the gear change. -
3rd - 3,000 / 58 kph - (1,987 / 38 kph)
4th - 3,000 / 80 kph - (2,195 / 58 kph)
5th - 3,000 / 102 kph - (2,320 / 80 kph)
3rd - 4,000 / 80 kph - (2,649 / 53 kph)
4th - 4,000 / 93 kph - (2,927 / 80 kph)
5th - 4,000 / 141 kph - (3094 / 93 kph)
3rd - 5,000 / 100 kph - (3,312 / 66 kph)
4th - 5,000 / 137 kph - (3,658 / 100 kph)
5th - 5,000 / 177 kph - (3,867 / 137 kph)
If you think of 3,250 -5,000 rpm being the powerband, and it might not be with your car, and if my figures are correct you will see only about 5,000 rpm in 2nd, 3rd and 4th will get you in the band on these figures. You do not have a racing gearbox or racing gearing so if you drive like that Felica guy you will be putting additional stress and wear on tour engine and transmission. Same for changing up just before the rev limiter, do you even have a rev limiter other than yourself, is your car capable of being pushed to the 5,000 rpm let alone more without regret.
A test on a level empty open road is fine but be sure what your car's limits are first, top fuel dragsters can go to 161 kph in 0.8 second but the engine is completely stripped down and built back up every 1/4 mile (400m) run and the gearbox, clutch and fuel are slightly different and even then they often break.
You do not need an oil gauge, you may want one but that is a different matter, it may help you or just give you more to worry about.
If the engine is NOT being laboured then 3,000 rpm all day long is fine but if you feel losses it can be because of other than or as well as engine oil, this is why I prefer a good quality oil in an oil car, a synthetic oil will offer more protection and for longer and deal with the heat (and cold) better generally but it can't make up for any engine issues (well perhaps a very little by allowing a limp home rather than roadside stop(s).
What is synthetic and what is 'synthetic' as a marketing term is a different matter, but a good oil is a good oil whatever it is or called.
For the gearbox with how you are driving the car and want to if you like Castrol this might be a better oil for you, Castrol TRANSMAX Manual Transaxle fully synthetic that's suitable for you but also as with the engine oil change the thotoughness of the change is important.
It was the 123ignition fully electronic dissy that I was referring to, I had the basic what they now call "switch", another Renault "switch" might include curves suitable to your car but you would have to enquire and note the LPG. -https://123ignition.com/products/page/2/?filter_brand=renault
The cost of it must be considered as spread over time and mileage and the improvements you could get (starting, idling, throughout the rev range, no farting about with points or messing around with timing once properly set).
Well that's enough for one night, if you can get a 75w-90 the 90 should be better at high temperatures particular a good 75w-90.