James was too unassuming to post this, but I did it any way, sorry James
Multigrades work by taking a base oil and adding stuff that reduces the
tendency to thin out with increasing temperature - i.e additives that
improve the viscosity index (VI improvers)
They are long chain polymers that are curled at low temperatures and
unravel with increasing temperature
In some research I did years ago in SAE transactions I found that these VI
improvers are subject to physical damage - loaded, sliding metal surfaces
can cause them to break, and the affect of them reduces over time and the
oil gets thinner at high temperatures. I suspect shear stabilisers may
have some role in reducing this affect
A monograde oil has no VI improvers and this whole thing is moot. There is
no mechanism I am aware of for it's viscosity to reduce with use. I'd look
for detergents - it's bad to put a detergent oil in an engine that has been
run for a long time with non-detergent oils (specially without full flow
filtration) as the crap deposited on the internals will get into suspension
and pumped round. Reverse does not matter and switching often between
detergent and non detergent does not matter.
Oils for gearbox use also contain extreme pressure additives - molybdenum
disulphide is an EP additive that you use as a build lubricant to provide
lubrication before oil is pumped round. They are intended to prevent
metal-metal contact even if 2 surfaces are pressed together till all the
oil is squeezed out (e.g. gears) and work by bonding to the metal surface
My opinion fwiw is that any decent monograde of SAE 40 or 50 will be just
fine and if it is specified for use with gears that's even better, and I'd
personally go for one with detergents.
Development of oils is all about trying to extend service
intervals. Changing at 1000 miles means none of the affects people are
working to eliminate are significant for a bevel (apart from VI improver
shearing) and probably the biggest problem is clutch wear - it wears slowly
for sure, but particles of friction material get added to the oil every
time the clutch slips, and the only place they go if they don't stay in
suspension or get trapped by the filter is the sludge trap.
So, as the bevel mantra goes... any decent oil, changed religiously!
