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142 Guy · Jan 15th, 2022, 18:29

A couple of points.

5W50 will not be 'thin' for the UK's climate. Under hot test conditions 20W50 and 5W50 will display approximately the same viscosity. That is what the 50 rating is all about. The difference will be at low temperatures where the 20 in the 20W50 rating may cause it to look more like Marmite versus the 5W50 which will flow better. If cold weather operation is not a significant issue then 20W50 will do the trick.

The blend of molecules making up a synthetic are not necessarily smaller than conventional oils; but, there may be a problem with lack of seal swell. I do know that when Mobil 1 was first introduced, oil leakage from neoprene / silicone crank seals was not uncommon on engines switched from conventional to synthetic because the seals shrunk. The synthetic lacked whatever caused the seals to swell. As far as I am aware, that is not an issue with modern synthetic oils and probably would not be an issue if you are running the OEM felt seals.

As a related observation, conventional motor oils get their extended viscosity index by the addition of Viscosity Index Improvers. Viscosity Index Improvers deteriorate / loose their effectiveness with age. Both 20W50 and 10W40 wide viscosity index conventional motor oil require more VI than conventional 10W30 or 5W20 so your more regular replacement of 20W50 is a good idea. Synthetics are a different can of chemistry.

Jan 15th, 2022, 18:29	#256
142 Guy Master Member Last Online: Apr 27th, 2024 15:46 Join Date: Jul 2020 Location: Saskatchewan, Canada	A couple of points. 5W50 will not be 'thin' for the UK's climate. Under hot test conditions 20W50 and 5W50 will display approximately the same viscosity. That is what the 50 rating is all about. The difference will be at low temperatures where the 20 in the 20W50 rating may cause it to look more like Marmite versus the 5W50 which will flow better. If cold weather operation is not a significant issue then 20W50 will do the trick. The blend of molecules making up a synthetic are not necessarily smaller than conventional oils; but, there may be a problem with lack of seal swell. I do know that when Mobil 1 was first introduced, oil leakage from neoprene / silicone crank seals was not uncommon on engines switched from conventional to synthetic because the seals shrunk. The synthetic lacked whatever caused the seals to swell. As far as I am aware, that is not an issue with modern synthetic oils and probably would not be an issue if you are running the OEM felt seals. As a related observation, conventional motor oils get their extended viscosity index by the addition of Viscosity Index Improvers. Viscosity Index Improvers deteriorate / loose their effectiveness with age. Both 20W50 and 10W40 wide viscosity index conventional motor oil require more VI than conventional 10W30 or 5W20 so your more regular replacement of 20W50 is a good idea. Synthetics are a different can of chemistry. Last edited by 142 Guy; Jan 15th, 2022 at 18:31.