What theee.... :confused1:

Synthetic, and fullsynthetic oils have been made and used since the 60's (used earlier than that for aircraft ect)


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Os for oilthicknes, a lot of different stuff can be used. i wouldn't go under a xxw40. The ticker the oil, the thicker the oilfilm between parts.

I use 5w40 in winter, and 10w50 in summer. Sometimes I use stuff in between, like 10w40 and sometimes 20w50 aswell.

All but one have been fulsynthetic. i just change my frictiondiscs in my clutch because the engine was given an overhaul as a precaution. The clutchplates were in spec, precisely in the middle of max/min meassures. Thats after 120.000km The oil cant be so bad afterall :scooter:

Ya , the myth that synthetic oil can be harmful to an older engine is one of those things that just keeps chugging along. The only time using synthetic can be bad is first fill on a new or rebuilt engine, then you do not use synthetic so the parts can seat(which ironically points to how good synthetic is!) . Probably many issues come from someone doing that and then thinking the oil failed.
The thing synthetic has that I like is for temp changes , it is way more viscosity stable than regular oil. That 10w @65F can be 20w @20F with regular but synthetic is maybe equivalent to 11w. I really notice it in winter when temps get to -20F, with synthetic my truck turns over fine, but with regular it barely turns and knocks (from lack of oil on startup) for quite a few seconds longer, over time that adds up to a lot of wear.

........................... the oilman has a view:-
http://www.horizonsunlimited.com/hub...er-bikes-42793

If you really must stick with mineral oil, Halfords used to have their own brand stuff; I last used it about 12 years ago, at best guess, (in a Yamaha XJ900 Diversion which consumes 1 UK gallon by the way) so it may not be on their shelves nowadays.
I recall checking the specifications on the container at that time and it was good stuff - it came in a very "mucky brown" colour plastic can which made it look very anonymous on display.

I have an old 43f xt 600 and was using 10w 40 semi , was talking to Mr Lambeth about something else and he reccomended 20w 40 or 50 for the older engines, i also checked the owners manual for the bike and the new clymer manual i got and bug**er me 'Mr Lambeth was right....... Both said 20w 40 so i got 5 litres of silkolene 20w 40 bike oil. My mate has a 96 xt600e and his manual says 10w 40 semi so i conclude that the newer bikes engines were built with semi in mind......no such thing in 1984 i guess

You are not doing anything wrong to the engine running 10W-40, remember they didnt have all the different thickness to choose from in the old days.

Newer bikes (same as cars/trucks) have tighter tolerances. Thicker oil doesn't flow into or though those spots so good and you can actually damage a motor by running too thick, especially at cold startup. If it is called for originally then the thicker is fine and MAY ( no way to prove it) be better in those engines, but probably only in hard use situations(aka racing). Newer engines, if 10w-40 is recommended, it is best to stay with it. The viscosity is one big reason why I like to use full syn, much more temp stable and flows similar whether hot or cold(think winter 10F), but regular oil will thicken up significantly in the cold and can starve areas of protection.

I'm starting to stick to 15w-40 for summer and 10w-40 for fall/ spring. In real hot months(80F +) I will top off with 20w-40, but make sure to change when temps cool down. This is in my XT600 and Honda XL 600 , both state 20w for summer in warmer temps.