This should get interesting. 
I am wary of modern gearbox oils because of the risk of the additives affecting the bushes, but for engine oil my reasoning is that it just isn't worth the extra cost. Most of us don't do vast mileage and the oil changes are quite frequent so mineral is good enough.
I have run synthetic in my Commando, showed lower oil pressure cold and higher hot vs the equivalent non synth, but it if you have a small leak with non synth you will have a much larger leak with synth oil.
Have also tried Synth gearbox oil in the Commando. Gearbox oil i change when the shift is no longer silky smooth, which tends to be more frequently that the manual says, and synth did last longer.
Also i believe synthetic is designed for modern bikes which are this new fangled unit construction so their engine oil sees more than just the engine, so i presume has stuff in it that we don't need and possibly don't want?
In both cases on my Commando i decided the extra cost wasn't worth it and went back to my old ways, which is where i am at with my 16H as well.
I use Rockoil RTM in my Commando and Dommie which is rated as a 20/60w oil and synthetic. It's a race oil and good for high temperatures. It's expensive at £28-£30 for 4 litres but I think it's worth the extra cost as I have trashed engines with lesser oils and cheaper than an engine rebuild. For the number of miles I do on each bike which isn't much, it's getting max protection. It's very viscous and sticks like sh*t to a blanket. I've been using 50w Rockoil in the singles and I'm thinking of changing to TRM for the same reasons. I know people use it in gearboxes but I haven't tried that. That's my 2 penny worth 
email (option): horror@blueyonder.co.uk
And here's my bit..Due to the high detergent content of synthetics, which leads to a lot of particles of carbon etc. in suspension, the general recommendation seems to be that a proper cartridge oil filter should be fitted when using these oils.
It doesn't apply too much to our old bikes with seperate engine, gearbox and transmission resevoirs but due to the inclusion of 'friction modifiers' in these oils (which are there to reduce friction) they are a completely unsuitable for wet clutches such as the Triumph clutch and will obviously cause additional problems if they get anywhere near an M20 clutch. Remember that if your M20 drops its oil into the crankcase it can leak into the primary and get to the clutch.
Finally, I have been told that for the same reason (friction modifiers/reducers) these oils should be avoided when running in older type engines as they actually inhibit the process of wearing in mating parts.
This would be less important in new engines which have close tolerances and much higher surface finishes from the outset than older types.
I have never used synthetics at all so cannot quote from actual experience... However, if you can get 50,000 miles from an M20 or similar engine on 'conventional oils' (and you can easily exceed that) then there may be a good argument for leaving synthetics to the engines that really benefit from them like turbo diesels and Suzuki Hyabusas..especially when the increased cost is factored in...Ian
email (option): ian@wright52.plus.com
The consensus in the Norton Commando world is to fully run in on non synthetic and only then consider the change over for that reason.
It's interesting to do the sums on this.
The AA used to do 50,000 miles on their outfits before they were returned to BSA for a top end overhaul..so I will assume a top end overhaul could be required at that mileage.
Using BSAs recommended oil change interval of 2000 miles and the 5 pint oil capacity of the M20 oiltank that works out to 25 oil changes...or 125 pints of oil.
That equates to 15.62 gallons. Currently you can buy 5 litres (approx. 8 1/2 pints) of straight 40 for about £15.
So 125 pints divided by 8 1/2 = 14.7 or approx. 15 tins at £15 per tin. That comes to £225.
Horror quoted a price of £30 for 4 litres of synthetic..125 pints is approx. 71 litres so at the price quoted that works out to £532 approx.
A difference of roughly £307.
If you are one of the very few people that might actually cover 50,000 miles it could be considerably cheaper to stick with the monograde and do the top end overhaul, specially if you plan ahead and buy the piston, valves, guides etc. now!!.(£120 should cover it)....and there is also the fact that even with synthetic oil you are still going to have to do that overhaul at some point..Ian
email (option): ian@wright52.plus.com
Good point Ian.
Actually, I can get 4 liters of a good mineral straight 40, API-SF/CD for about 10.8 UKP,
Which even have a MIL-L-2104D standard.
(The green goo foaming oils are cheaper, but you might as well put in a shampoo…)
So, If one can expect to get some 50000 Miles / 80000 Km on it, it would take be about 75 years to get there,
In my current pace…
email (option): noam10@gmail.com
John, I understand the difference between "running in" and wear. When an engine is first run after a rebuild, there is alway metal in the oil as it beds in. But if an engine runs in slower with synthetic oil, it shows it's good at it's job, which is the point of this discussion. I usually run an engine for 50 miles and change the oil, then 500, then 1000. This is when there is metal in the oil and I want it out as quick as possible. Maybe I'm over cautious these days but it's the things that have happen to me with engines that make me this way. I would rather buy another gallon of oil that do damage to a newly built engine.
Maybe I've just been unlucky, but I have changed so many cams etc on twins that it can only be down to the oil. I always used 20/50, Castrol GTX and Duckhams, then Valvoline. Although I have never had a big end go (eeeek that's tempting fait 
) but as I said before on another post on this subject, I completely ruined a Commando engine in 2000 mile from a rebuild. There is no other reason than the oil which was Valvoline and a local enigineer confirmed that's what he thought. I went to Antig's who build speedway bikes, JAP engines, and other big singles, and asked them what oil they use. Obviously there were the caster oils which don't mix with other oils and aren't now the best, but do smell nice The oil they use is Rockoil TRM, after reading more about the oil it does seem right for me.
I do agree with you Ian, for a BSA single ridden normally it is probably over spec than required but certainly wont do it any harm if it was used. In fact if what everyone has said is correct, synthetic oils can be left in engines longer than mineral oils, clean engines better so no sludge build up, and resist wear better than mineral oils. All this for a few quid more a gallon. What's not to like
email (option): horror@blueyonder.co.uk
I agree with your sentiments Horror...and wouldn't question your experiences with different bikes and oils.(though poor or borderline oil supply might be a factor as well as the oil itself..early BSA twins suffered cam failures for exactly that reason)
In relation to my own bikes it's a bit like saying are we 93,000,000 miles from the sun or 93,000,001...the extra mile makes no practical difference.
The majority of viewers on this forum are unlikely to do 70,000 miles or more...as Noam said it would take him 75 years. So in practical terms there is no difference...UNLESS the use of synthetic oil prevents problems with a particular engine in the short term or an engine is being subjected to more extreme uses, such as racing, where due to the much higher loads benefits may be gained.
I certainly have found no need to improve the oils used in my BSAs as they haven't had any failures that indicate a problem, but that doesn't mean a Commando wouldn't benefit..they are completely different engines after all.
The limit of my opinion here relates to BSA engines where I feel any gains would be marginal or theoretical, as the engines are reliable and durable on mineral oils over higher mileages than most people do these days. That leads me to conclude a more than doubling of the price of the oil used won't really return any practical benefits in that application...Ian
email (option): ian@wright52.plus.com