The original Honda,the 600 was actually a motorcycle engine and trans fitted into the chassis. The first civic when it hit the market in 1972 also used engine oil to lube the tranny. Do keep in mind the engine and trans do not share the same oil supply.

for the bike
Motul 5100 4T 10w/50,
I think you would need bike oil if one had a wet clutch. [true ?]

Personally I would prefer a 20W/50, old bikes have big clearances vs the newer bikes, so the next change will be a 20W/50. I am also having some case leaks, but most likely unrelated.
Might give the Belray a try....

for the car, porsche 1987 928s4
Penrite SIN or HPR 30
these have a very high zinc content, which is very important for 928s to stop cam shaft wear and thrust bearing failure. Please don't argue this, its a world wide fact!
Again anything under a 20w for out temperate climate is a no no due to the larger clearances.
running a 5w/30 or 40 causes problems, where a new 911 is designed for the smaller clearances.

I have never heard of zinc causing a sludge, oil is a cheap engine insurance, even missing a filter change for every second oil change

I gather thats ZDDP.

Interesting, but not conclusive thats it ALWAYS bad. Metal to metal contact, like cams lobes and the thrust bearings, problem areas in 928 motors.

There are plenty of 300-400,00 mile 928s around in the states that are showing no signs of wear using the correct 20W/50 high zddp oils vs destroyed motors using low viscosity oils. the bearing clearances are just too large.
Some racers have excellent results with Amsoil.

My personal opinon is that you just need a good 20w/50 oil!

Mobil 1 delvac, the diesel oil had high quantitys of ZDDP.

Amsoil is almost non existent here. I remember the Amsoil rep coming to an Alfa meeting in the 80s! I used their synthetic gearbox oil in my Alfetta, gerabox at the back, so slow to warm up and poor 2nd gear synchros, the oil helped a lot.

I do beg to differ about bearing clearances needing an appropriate oil for the conditions. Again , put a 0 or 10w oil in a 928 and it won't have enough oil pressure at idle on a hot day. My comments apply to our temps here where it does'nt reach 0c in winter.
As I understood it the bearings run on a film of oil, so while a 10w oil may be ok in a northern Euro/US winter its too thin on a 40c day.
Now I gather film strenght should prevent metal to metal most of the time, otherwise the motors would sieze in a few minutes, but not long term in our 928s.

Isn't ZDDP also being reduced in the latest spec oils as its poor for the enviroment? You mention cats.

What about redline and I think its called royal purple or something like that?

Sorry, but i dont have enough time to read all of this disscussion(this topic), but if someone have some evidences about using WRONG viscosity/brand, etc oil in Paso engine; i am very intresting to hear it HERE and what were damages, maybe some fotos , etc....

I exclusively use Mobil 1 oils in all my vechicles.
I wouldn't use Amsoil in my lawn mower...

i dont have a lawn mower but did put fuchs silkolene comp 4 in my paso last service and will probably do so next time.

lawnmowers at dawn.

lawnmowers at noon. high noon. so did we end up with the answer to the original question? what are the best oils for use in our bikes?

nope!
As many opinions as there are a** holes And I only mean that in that there every one has an opinion ..no offence mean't to any one...having to explain a joke so you don't offend sort of defeats the joke?

now using Penrite SIN 20 a 20w/60 full synthetic, motor hasn't siezed yet!